Can BRM prompt staff to allocate a specific bike when a booking checks out?

Yes — this is the Allocate on checkout setting. It only matters if you use GBAL (Generic Book Allocate Later): a booking that's still generic — a Product Line row rather than a specific bike — because nobody's picked an exact item for it yet.

What it does

With the setting on, if a confirmed reservation still has unallocated (generic) items when staff press the next stage button to check it out, BRM stops and opens the allocation dialog there and then, prompting a specific item to be chosen before checkout can proceed. With the setting off, staff have to remember to allocate manually at some point before or during checkout — nothing forces it.

Either way, it's the same allocation dialog described in Allocating Items from Product Lines — this setting only changes when it appears, not what it looks like.

Where to turn it on

Settings > Administration > Features > Allocate on checkout. It's off by default.