I am not sure it's reasonable for you to expect the dealer give a loaner during these:
1. Car out of warranty
2. It's a routine service (not a breakdown).

That being said, a dealer should offer you a loaner if a problem occured, regardless if the vehicle was brought from that dealership.

When I took my e70 for some issues, I was offered a loaner (sweet '11 335d), despite the dealership not being the purchase place of that vehicle.