I had somewhat of a small breakthrough several minutes ago. Grammatical percussion is really all about subdividing time. It's about taking a given chunk of time and doing something with it. Unlike the melodic grammatical system, the percussive system should have "words" that correspond to different ways to subdivide a set amount of time. This way, one does not have to deal with overlapping words and other problems that arise when the length of a word isn't absolutely defined.
The process of subdivision is recursive, so the process might start at the level of a measure and ask, "ok, how do I want to subdivide this measure?" The response may be, "I want to equal subdivisions." The process will then make another decision: "Do I want to go ahead and replace this chunk of time (half a measure) with an 8-beat word? Or do I want to subdivide it further?" Let's say the computer decides to split the first chunk of time into two quarter notes (4 beats, also 1/4 of a measure). It decides to replace the second chunk with a word.
So the system really operates around two entities and one process: time "chunks ," words, and subdivision, respectively.
Conceptually this is a big step forward for grammatical drumming. With any luck it will make the process a lot less painful.