Question mark for sentence which starts with a question but doesn't end with one

The usual mantra is 'A parenthetical is deletable without the matrix sentence's syntax, usually expected punctuation [and certainly basic meaning] being compromised'.

Few would object to

Can you book a room in advance (because otherwise we'd be in trouble)?

but this changes the emphasis from that in the version using the comma. But you could argue that the same punctuation convention should logically apply when the comma is chosen to offset.

Modern styles probably allow even a sentence-medial question mark:

Can you book a room in advance? because otherwise we'd be in trouble.

But the use of a sentence fragment is probably less controversial, and quite acceptable as an informal option to many people nowadays:

Can you book a room in advance? Because otherwise we'd be in trouble.