I'm playing a VH11 on a TD12 and haven't found a perfect solution as
yet either.
Basically did the same thing you did.
Attempted to adjust the footsplash on the TD12, made sure no foot
controller in the yellow, changed the threshold.
For the most part it works. Works better in Rockband than in GH.
An alternate work around - if you tend to mostly use the rim hit on
the HH is to not program in anything but rim notes.
Not the greatest but worth a try.