How can I open and read a drum map file

You might want to have a look at the map files contents using a hex editor. 

Using Cakewalk's drum map editor, create small drum map file with, say, one map entry and then, using a hex editor, see what changing values does to the internal contents of the file.

Depending on your programming skill level, you could create a program (maybe in MS Visual Studio) to programmatically extract the relevant information.  The note names are in ASCII with each character separated by a zero.  The note numbers will be in hex, so you would have to convert it to decimal.

The trick is the use the hex editor to see what actual data Cakewalk's drum map editor produces.

Long winded process I know, but unless someone has already done the above leg work, this is really the only way to do this.

Thanks for your ideas and knowledge. Seems like there is no quick and easy solution. Since this is something I just need once and don't want to dive too deep into, I think I will really just copy and paste from the SONAR piano roll.
I actually downloaded a hex editor to see what it is and does but for now this seems to go too far ;-)