This is what I posted on Lukss12 thread and found this post too where Lukss12 says "It's necessary to analize the way Ubuntu mounts the partitions to create a proper ramdisk patching for ubuntu kernels....requests like booting non Android ROMs will be implemented!" Well that sounds like music in my ears.
I've been trying to get this Android method translated to UBports to get a view how UBports mounts partitions but my knowledge is insufficient. Does anybody know (a better way) how or where to get this information?

Hi Bastos,
Dual / Multibooting also made me a linux user in the end. I also think it could gain more smart phone users to try UBports. There's already a Beta EFIDroid for Nexus 5 hammerhead:
https://forum.xda-developers.com/google-nexus-5/development/multiboot-efidroid-nexus-5-t3580493
In the FAQ on the site of UBPorts is mentioned that multibooting may change with Halium project, but that’s not carved in stone yet and they won’t make any promises.
Good to know that the Fairphone community is having their thoughts on this. Is it correct that you are already experimenting on this? It looks very promising.
So I do hope some day the UBports community will also come to a point of thinking of multibooting someway.

@elastic Same here. I own a Nexus 5 too as a daily driver looking for the right moment to give the Beta EFIdroid a try (to many ideas, to much to do and a lack of energy). If I can find the time I will post the results here,

@elastic Installed Beta EFIDroid on lineage on Nexus 5 to and indeed it installs flawlessly. Added a new slot in the EFIDroid Manager app for UBports. Next step is to intall a custom rom by rebooting and when EFIDroid comes up select TWRP, then select the newly created slot.
Go to the "Wipe > Advanced Wipe" menu. Select "System", then swipe to wipe.
Finally you can install all the zips you want on that slot. TWRP needs install zipfiles.
The UBports server does not provide these zip files. That's were the process stalls.
Maybe there is a way to manually create this install zip file for UBPorts?
Any leads are appreciated. I'm no programmer or developer. Just trying by guessing and trail and error.

@elastic Is your drainage pump working again?
That's a clever attempt and thanks for posting. My wild guess is that the process could work in a situation where you have UBports installed as a second OS in Multirom. A few months ago I found a Multirom version for the Nexus 4 mako that can do this: https://forum.xda-developers.com/showpost.php?p=72455220&postcount=2140
It works pretty well but uses an old recovery and there is no maintenance so i dropped it. I couldn't find a similar Multirom version for Nexus 5 hammerhead. Maybe other readers may know if it exists and where to find it.
Just to see if we can get UBports working in EFIDroid.
I'm looking into the UBport tar-files trying to get a view and understanding how the flow of the proces of installing UBport might be.
Updated LineageOS to latest version today. All went well. Had to reinstall Supersu (of course, duhh).

@elastic Well that's a pity UBports won't boot while other custom roms do. There must be something we are overlooking here.
I tried replacing TWRP recovery with UBports recovery. First on my Nexus 4 mako with only LineageOS installed. No problems. LineageOS boots and UBports recovery shows up nicely after booting into recovery. I thought this could be a step forward.
So replaced TWRP recovery on my Nexus 5 with UBports recovery from Version 3 and started LineageOS and EFIDroid. Chose install/update in the menu of EFIDroid. It showed a big Repair button. Tapped it and it reported that the recovery only has an ESP backup. Repair completed and then i tried to reboot into recovery. That gave an error: 'Previous boot failed' and another error: 'init returned'. Played around a bit in EFI bootloader but it keeps loping back to EFIdroid bootloader just as in your method.
Reinstalled TWRP and I'm back to where started.
Maybe I have to try this from a clean install (without replacing TWRP) and also as a comparison trying to multiboot Canonicals Ubuntu Touch.
My guess is that UBports needs its own recovery and there is something special with the boot process.

@oli-sax @elastic I posted this before in category support link text, edited the dualboot script to get images from ubport instead of ubuntu and then sideloaded the ubport files. Release 13 of channel RC (not the latest release) got downloaded but no installation is started. I guess it lacks a installation script or binary. My other guess is that this feature is not fully implemented just as the option to customize the pull server.
I'm gonna try to sideload Canonicals ubuntu files and see it that leads to a install procedure.
I will post some more details of my experiments later.

@elastic Did some experimenting on my Nexus 4. Installed UPports and then flashed TWRP as a new recovery. All functions well so TWRP and UBports go wel together once UBports is installed. But to install UBports and to update it UBports recovery is absolutely necessary. So if in EFIDroid UBports won't boot as you tried various times my guess its because of the boot.img. The question is not only to get support for UBports recovery in EFIDroid but also to for its boot.img. Did not have time to make an account on XDA forum. Needed to clean my laptop (Samsung N510 running Xubuntu 16.04) too. Restored a Clonezilla image from a month ago and updated the system. Now I have the NVIDEA driver and configuration back and plenty of free disk space. Tomorrow will be a busy day so it will be no sooner than after dinner to make an account on XDa forum.
You are driving a lot of kilometers for your work! I'm hoping for you that there are not to many traffic jams.
cheers to you too.

Just registered at XDA forum and posted in Lukss12 thread. Now hope for the best.

And this is what I like to read too!!! Multiboot different recoveries in EFIDroid