It seems to me that in order to do this, Line 6 would have to release the code for the M series firmware. That is not likely to happen. Also, the hardware of M13 is greatly involved in how you see the data, so it would seem to me that your "Editor" would have to emulate the hardware components in the unit. If you had all that anyway it wouldn't really be a leap to be able to edit onscreen, but as I said, I doubt that Line 6 would release the needed propriatary elements...

I put in a feature request around 9 months ago asking for the format/schema of the sysex files but never received a response.  I volunteered to build an editor as well (software architect, 10 years).  No luck!