Just a FWIW,I tried something similar with a GPE MOTO G & T-MOBILE's WI-FI Calling.
I found an .apk download for it,side-loaded it & yes it loaded up,but,it didn't work,only force-closing on start-up/boot.
Even if someone can pull the .apk from an AT&T variant of the same hardware,I'm guessing the firmware/software is deeply imbedded & even if it can work,it'll probably take quite a bit of doing.
Also,I just went to AT&T's site to look for the HTC ONE M8 & add visual voicemail to the cart & it wouldn't allow it.Says this phone isn't compatible w/visual voice mail.
Might wanna make a couple calls/visit AT&T to verify.
Edit: Took another look @AT&T's site,visual voicemail is available on the HTC ONE M8.
Please keep us posted on your progress/results w/the Developer's EDITION & visual voicemail.