You can
already execute any MCC command, MCWS URL, or script with the built-in system.
1. Open
Tools > Options > Remote Control and open up the Remote Control Commands Manager (click the
Commands link near the bottom of the dialog).
2.
Add a new
Custom type command.
3. Give the Command a friendly name.
4. Click
Add Run in the Command Actions section.
5. Execute whatever you want.
To run MCC commands, call the
MC Command Line launcher (mc##.exe) with the
/MCC flag. You can also call any
MCWS command using the /MCWS flag, which will work on the local machine even if Media Network is turned off. If you want to do fancy conditionals and stuff, call a VB script or something similar.
I was referring to your requests to have built-in conditionals and other advanced logic. You can do this with what MC provides now by executing scripts, of course. Or, if you don't want to write everything yourself, using a tool suited to the task (like Girder or EventGhost). Similarly, getting even one of those kinds of tools to handle RS232 commands can be a major undertaking. I don't know that it makes sense to include this in MC itself.
It is, after all, a Player and library manager, not a home-automation utility. It has some capabilities, but I don't know that integrating something like Lua and RS232 drivers or whatever is where they should spend precious resources.