This is something that came to mind over the past weekend here when it was raining and especially as my mom had to drive my car around for a bit. Normally not something that bothers me much as I'm fully aware of it but anyone here can attest to the weird braking performance in wet weather which is to be expected in hybrid vehicles. Regen basically has no difference in feel regardless of conditions and when you get down to ~5mph when the friction brakes kick in if they haven't been used yet, they'll grab something fierce if still wet. This is especially noticable at low speeds in town where the friction brakes are used very minimally.
Now BMW has a useful feature where when the rain sensor is detecting a decent amount of rain, it will activate the brakes just enough to bring the pads right up to the rotor in order to assist in brake drying without actually causing any drag. I seriously think this would be a useful feature in hybrids in general (or at least models like ours that seem to take more advantage of regen than others) to keep the brake feel consistent regardless of conditions. Though I'd wager BMW probably has a patent on this somewhere making it difficult for others to implement.