Please correct if Im wrong, but I believe ABS should be built in such a way that the bike will never be totally without brakes, no matter what? I would believe that requirement would be compulsory for any manufacturer who builds such systems.

So, taking the ABS motor fuse off (or any other fuse thats related to the operation of ABS), should resume the system back to normal brakes. Im not 100% sure, because I havent tried.. Ive got an ABS bike, but its not BMW.

Please also note that if you plan to ride for an extended period of time with the system deliberately disconnected, the ABS motor could be damaged. This is because the hydraulic pistons inside will need to function every once in a while, if they remain stationary all the time, they might get stuck. For this same reason, they will function one time back&forth every time you put the ignition switch to "ON". Even when ABS is on and functioning, these pistons may not get to work, unless the user activates the brakes hard enough to activate ABS, and this might not happen very often.

Or at least thats what happens with my bike, cant be 100% sure how BMW works, but I´d guess its a similar system. So would be a good idea to reconnect it every once in a while, even if its somehow disabled most of the time.