EPM Support for Grandstream HT-8xx series?

I’ve purchased an HT-814 to check out, and to my surprise it’s not listed in the commercial EPM module. Checking the Wiki, I see that the entire HT-8xx series isn’t mentioned at all (not even present as tested or not).

Is there any mechanism to know if/when new device support is added?

Click the ‘Watch’ button at the top of the wiki page and you will get email notifications of changes.

Is there any way to determine what plans are in place to support specific models of devices?

Not without pinging the vendors. Specific phones are managed by the vendors, not Sangoma, so if you have requests, make sure to bug your phone vendor as well as checking in here once in a while.