Is it possible to cut down a Roland GK-3B to fit on a Rickenbacker foam mute plate?

The GK3B pickup has 6 separate coils and you'll only need 4, so I think it would be possible to cut down if you were super careful and you'd need to do some splicing of very small wires. I don't know what they look like internally, but I'm sure there are pictures somewhere on the net to give you an idea. Edit- found this picture of the GK-2 guts online just for my own curiousity, I'd guess the GK-3 guts are similar: .
There are options out there for other "multicoil" bass pickups (ie pickups with a separate coil for each string), but the only other ones I am aware of that can be made directly Roland-compatible without custom wiring the 13-pin output yourself are piezo pickups (via something like the Graphtech Hexpander). This actually probably wouldn't be too difficult to cobble together yourself if you are patient and good with soldering- the best bet would probably be to have a custom pickup wound for the purpose given the small cavity, tight Rick spacing, and very small coils generally used for these pickups (the synth units must want to see a fairly wide range signal). There is also the Fishman Tripleplay system, but I believe it will also be too large for the mute cavity.

In my opinion it makes more sense to just mount the Roland pickup on a standard bass with the included patch-box or route out a beater for the internal controls than mess with a Rick (particularly a beater bass, as the actual tone of the bass as is would be inconsequential when using the synth output)