Does anybody know if it's possible to add a message, other than the copyright with the 80 character limit, to a custom Garmin map that would appear on the GPS unit?
Different units show different info. It could be map name from img header or MPS subfile, copyright from TRE subfile, license from LBL subfile. Garmin maps in GMP format contain some additional legal warning, which I have never seen presented in GPS.
For cgpsmapper copyright is fixed but you can set own license text. For mkgmap you can set copyright and license. Map names in img can be edited by GMapTool.