I have also managed to get it working with the modified APK. Steps I took:

• Install GApps
• Install Pokemon Go from APK
• Create Pokemon Trainer account
• Open Google Maps (needed to get GPS working properly)
• Open Pokemon Go and log in with the trainer account.

Something I discovered while doing this is that the password box when you are creating an account silently truncates your password. If you put in a password longer than the max it allows (16 chars, I believe) then it will accept it, but you won't be able to use it to log into the app (which does not truncate input).