I did like @LoFiLeiF and looked at some of the documentation on the Boss website, I found this extract:
“As an example, you can send program changes or control changes to select patches on multiple connected MIDI devices at once”.
Midi Guitar is a midi device therefore you can control it.
ES-8 seems to be quite a complex device, I think you should read carefully the manual to understand how it works and how it fits into a midi environment.
In addition you can connect many expression pedals and footswitches to the ES-8 which can give you much more control in Midi Guitar than just changing the patch.
For more info, check out this post: MIDI Guitar 2 and expression pedals