Another truly open alternative in crowdfunding stage (I really wish it will reach its funding goal): https://puri.sm/shop/librem-5/
Plus, you might just be able to use anbox to run signal anyways. https://anbox.io/
It's far from a perfect solution, but I feel like it comes closer to the mark than Signal.
https://www.reddit.com/r/GPDPocket/comments/6idnia/linux_on_...
Oh, nevermind, they take Bitcoin, which means other people paid for my new phone :-)