I know I've asked that before, even emailed Brooks; no reply.
I did not take the plunge to try it out.
Others use skateboard tape, ranger bands, bike tire tubing, or other. I currently use the small sized Butler Creek grip.
It fits REAL tight with the medium backstrap, & does not add much to the original grip size.
yep, i had AGrip from a HK USP that had some bad adhesive and used it on my P99 (while i was getting a replacement) and it worked fine. if i were to do it again, i'd Agrip it w/o the back strap on, then keep the backstrap uncovered. i haven't done it again because i have a P99MIL.