Can you service (i.e. fully remove and replace) a Square Taper English Threaded Bottom Bracket without removing the chain and/or front derailleur? Ditto for other BB types if the answer varies by BB type.
Picturing it in my head, the drive side crank arm will take the chainrings and therefore chain with it as it is pulled, but the front derailleur wants to keep the chain where it is. If you manually pop the chain onto successively smaller rings, and then right off the smallest one as you pull the crank arm, then I think the chain will just hang off the derailleur, out of the way, giving you full access to the BB.
Am I missing anything?