Hi Robert
This is a response to your suggestion that a request for Memeory Map Icon support be placed on the GPSBabel mailing list.
I have MM and am happy to help in any way that I can. I must point out that I'm not a programmer but 'merely' a user of the s/w package.
What I can tell you is that the latest version of MM now supports the addition of custom icons. This I currently do by using the XSL script kindly provided by Roolku on the geocaching.com forums. This (in my case) produces a *.csv file which is loaded as an overlay to the MM program. I can provide you with examples if that is any help.