Ha, I've solved this on my own, and it's easy:

Use an .xml editor, open `\mGalaxy\Data\BaseSystems\BaseSystems.xml
Add the retroarch lines for each new core within the <emu></emu> code block of the [MAME] <system>
-I copied one of the lines from the fba <emu></emu> code block and replaced the fba ...

Retroarch isn't a selection as an Application for the MAME system... am I missing something?  It's there for everything else.