I personally would give it a try if you're really curious. But, if you haven't ridden your tandem solo, just beware that it'd feel funky with a good frame. And if the temporary fork has a different rake, that'll add to the funkiness too. I'd probably try riding it with the full team just to check it out. Certainly no killer record setting ride, but... :-)

I'd think it'd be visually apparent if the frame were damaged, but I agree, the advice of the LBS is the standard CYA answer.