Scott, there is no direct way. The workaround is as follows:
Create image files of your button icons you want to use
Drag them into the source repository of the mimoLive document you are working on. This will create static image sources in mimoLive. (Best practice: collect them in a folder for better overview.)
Now open the Remote Control Surface in a web browser
Switch to edit mode in your remote control layout
Find the button you want to change the icon and click on the action gear
Set up the “Video Preview” option to show the content of your previously imported static image source
Switch back to “Live” mode for your remote control surface