So, as nobody jumped in, I did it and wrote them this email:
First of all, thank you very much for what you are doing, the world deeply need free and respectful software.
I read your call for suggestions for the HPP list and I saw the "Free phone operating system" category.
I'm a user of Ubuntu Touch for 2 years now as my primary device and I'm very happy with it, that's why I want to suggest to support that project.
I don't know what are the requierments to be accepted in the HPP list. I know (and agree with) the FSF philosophy of rejecting proprietary software. Ubuntu Touch is of course a free software project, distributed under the GPLv3. It is using Halium to be able to run on phones which requires proprietary firmwares, but is also able to run on plain GNU/Linux phones like the PinePhone, so without proprietary bits involved.
Do you think it is a good idea to include it in the list?
We'll see what goes out of this.