AFAIK heads (under pureboot) only works with grub at this point but it is a linux environment so booting kernels on EFI seems feasible. I see mentions of efi in the source code. There is a dependency on grub.cfg which bit me for months because the OS installers were not generating grub.cfg if the grub-install to MBR/partition was skipped.
I came across this thread bc I also wanted to use heads/pureboot with the librem key and Qubes with LUKS. This thread is the only place to find information on using Qubes with Pureboot and the Librem key. As you are experiencing the normal Purism response is lacking. Though they say that they have docs for pureboot, the docs are inaccurate and incomplete in a few places, repeated requests (to clarify, fix, or update) from myself and others have been ignored. My Librem 15 was unusable for half of 2020 and I wrestled with PureOS for the other half. I like the Purism philosophy and strategy but I fear the company is too small to effectively execute its ambitions at this time.
These reasons led me to improving docs with the upstream heads project. The default standard at heads is Qubes. I am adding OS install guides for other distros–including PureOS.