Hi !
Monticello is a distributed version control system, so you could have your own repository on smalltalkhub to commit your modifications. In fact, given how MC (doesn't) handles branches, it's probably the recommended way of doing if you want experimental/development code to be clearly separate from the official branch.
In terms of getting your changes merged in the official release, I'd suggest discussing them on one of the Pharo lists, or on the Slack channel. Some of the owners of the Magritte and Pier repos on sthub and recent developers are active there and should be able to help and even review your code or add you to the contributors.