Cool! Do you have any blog post explaining how to do it?
Isn't that a good way to get a modified / malware version of Signal? Anyone can upload to APKMirror, can't they?
This is also useful: http://www.onyxbits.de/raccoon It's a dektop play store client. I tried it out a while back and it worked, don't know about now.
Edit: The series is a little out of date, but should give you the general idea. ROMs that are probably new since that article was published that have no google dependencies are Replicant (entirely libre), OmniROM, Copperhead OS, and CyanagenMod w/ a no-gapps-script.