The real-time multiplayer API in Google Play games services is very feature rich.
An extension to support all its features would take quite a bit of time and money to produce but overall is achievable as a GM:S extension.
If time and money is a factor, you can always tailor the extension to support only project specific features, which would reduce the cost and workload considerably. Definitly a factor to keep in mind before seeking a developer.
You can read up on the Real-Time Multiplayer API and it's features on the
Android Developer Website, they also provide the basics of
implementation which gives a good indication of low level back-end RTM procedures.
hmmm, You could always submit it to YoYo as a Feature Request (via extension). It could be a popular paid extension.