I don't know any way to get custom symbols into an Etrex. Not only that, but as far as I can tell, only a very limited subset of all the symbols available in Mapsource, are in the GPS. The blue cross 'hospital' symbol, for example, just shows as a red blob on the GPS screen.
Naming waypoints as Paul describes, is good, if you have the time and inclination.
I don't find any problem with dashes.
Id can just about be done with 2 chars (36x36 is, er, a lot), then a dash, leaving 3 chars for the instruction. Actually 99.5% of instructions only require 2 chars anyway. I use things like:
-X3 (implies a roundabout, 3rd exit)
-LR (left then shortly right)
as well as the more obvious ones