Sending custom commands/params to stream plugin via JSONRPC

[elParaguayo]

Describe the bug
Not necessarily a bug but didn't want to submit as a feature request until I'd raised it here first.

I've been writing a stream plugin for a custom audio stream (streams audio from a TVHeadend server). I'd like the ability to change channel via the JSONRPC interface but can't find a good way to do this.

I tried adding "params" to the "play" command, but these get removed.
I tried sending a custom command "playChannel". That was rejected as an invalid command.

I could use "next"/"previous" but, with 100+ channels, that's far from ideal.

So, is there a way to specify a particular source to play?