I think I know how you would do it, but I don't know what command to use to initiate the "open voice search" key press. Maybe someone with more experience can answer that.

Here's how I would try to do it though. I use the Xonfluence skin, so that's what I will use as my exmaple:

Navigate to ~\com.semperpax.spmc16\files.spmc\addons\ and open your skin folder (ie. skin.xonfluence)
Navigate to the 720p folder and open Home.xml

Now do a search for "global search" and find where the button code is located.
After you find it, look for where it says:

<onclick>RunScript(script.globalsearch)</onclick>

You can run multiple <onclick> commands, so right under this line you could put another <onclick> command that initiates the voice search key press after first running the onclick command above.

Hopefully someone else knows how to initiate a key press through an OnClick command. I know that the IP button in the Global Search virtual keyboard opens the voice search, so maybe it's possible to create a command that automatically clicks that button. Not sure.

This is an awesome question that I simply can't answer. I can't even figure out how to voice search in SPMC with the Mi Box remote. I'm definitely gonna have to set up a search shortcut though. Good idea.