i downloaded the default software posted at vexlabs this morning and scanned it over lunch and on the train home. it seems the master microcontroller posts pwm data to the user microcontroller every 18.5 ms. this is plenty of time for six one to two ms pulses. the fact that the variable names have the letters p, w and m only increases my confidence in my theory.
unfortunately the source code for the master controller is not posted so i can't claim absolute certitude.
anyone interested can download
http://www.vexlabs.com/docs/VexStartupCode.zip
for now i'll be hiking to the vet for medicine for my hypertensive cat.
ta-ta