PostGIS development planning happens via a Trac instance.
In order for proper scheduling of contributions acceptance/merging it is recommended to file a ticket there and add your patch or a link to a patch or to a public git branch where your development is ongoing.
Filing a ticket requires setting up an OSGeo account, which lets you contribute to every project of the Open Source Geospatial Foundation.
As we understand you might not want to setup a new account, simply filing a pull request on one of the PostGIS code mirrors is also welcome, but it is easier for such contributions to miss our radars unless it also has a matching Trac ticket.
If you intend to be involved for more than an occasional patch we recommend you to subscribe to the development mailing list.