Not the exact situation, but similar. I was on a DL BE flight, sitting in row 1, when we were delayed on the tarmac--- about a half hour or so. The good FA's kept the pre-departure drinks flowing and consequently yours truly had to desperately use the bathroom. The realization dawns just around the time the captain makes the announcement that we are finally ready to depart and that everyone needs to be in their seats. I waited, and waited, and waited but the plane didn't move. Finally, I had to go, the feeling not made any easier by the realization that even if we started taxiing at that time, it would probably take another 15 mins to take off (this was at JFK AND if we were lucky)! One of the FAs moved to do something, so I quietly got up and asked if I could use the lav. She called the cockpit, and I was told it was ok.

While I was in, the aircraft started to move and the captain began making announcements to the effect that we were now really ready to go. So basically my being in the lav didn't change any plans.

It took another ~45mins, before we were airborne.