Bluetooth is made to transfer information. It can easily transfer the pressure information with the current position(s) on the trackpad, so really just three integers instead of two per frame. The security issue with Touch ID doesn't really seem unsolvable. The obvious idea is to just process the whole fingerprint information on a chip on the trackpad itself, so that the information about the fingerprint never actually leaves the device. But of course, security needs to be tight with this thing, even though I'm pretty much 100% sure that there won't be any Touch ID on the trackpad.
Admittedly, I am not sure about what's possible, but just judging by how much the secure enclave was talked up when Touch ID first launched, I envision Touch ID on a Mac being a part of the Mac itself and not a Bluetooth accessory --- as much as it seems to logically make sense. Adding the security chip to the Magic Trackpad/Mouse itself may force it into a price point that doesn't seem worth it, but we'll see.