Not sure what you mean by control commands…but bpm, sample count, etc…are part of the VST protocol.
I’m not sure if this would serve your needs, but it would cost nothing to have a look at Bidule. I don’t know that it supports MTC, but it still might be what you’re looking for. It ‘could’ play a SMPTE stripe (the kind we used to put on a track of analog tape decks to sync stuff up) that gear which can understand this LTC code can sync with. Software and hardware exists that’ll convert that old analog LTC to MIDI/MTC.
Bidule provides building blocks where you can supply or get information such as asio sample count, time signature, bpm, samples per ppq, last bar started, if the host/transport is playing or stopped, cycle start/end positions, looping status, etc.
These sync pins in bidule correspond to VST protocol information that hosts can pass along to plugins. Not all hosts send all that info, but…the protocol supports it, and Bidule can snoop and use or otherwise manipulate those signals in real time.
It can provide the master clock, or it can get it from the host when run as a plugin inside said host (if it provides the information according to VST protocols).
It has bidules to convert MIDI, or networked OSC to VST controls and vice verse.
It’s a fascinating Swiss army knife of a MIDI/AUDIO tool!
Perhaps you could host your instance of Groove Agent inside Bidule, and get remote control over ‘tempo’ from there.
I.E. Have a transport in Bidule. Sync Groove Agent to this transport. Then bind MIDI or VST events to manipulate the Tempo/Time Signature at will.
You could also have bidules to ‘switch’ among sync sources at will I think. I.E. Have more than one transport running, and be able to change which one your GA Instance is synced up with via remote.
If you have the plugin version of Bidule, you can also sync to the host itself. A good host will constantly provide tempo information to plugins. With bidule, you can snoop and manipulate those host sync signals in real time to stuff hosted in further down inside Bidule.