from the standpoint of damaging the amp ..... yes you could do it without harming the amp.
But playing bass thru guitar speakers CAN damage the speakers.
And especially if it's only a 100 watt bass amp, you're likely to get into clipping which is even harder on speakers.
You COULD blow those git speakers.
Concour. I tried a bass amp thru 4 10's in an Ampeg VT-40. Sounded HORRIBLE, I didn't even wait for the first note to stop reverbing in the room before I quit THAT rig...