In my experience of budget airlines, it is very easy to get an exit row seat even if you board last. This is because, despite what you might think, 90% of pax don't seem to realise that certain rows have more leg room.

Even better - most budget carriers board from both ends. This means that the last seats to be taken are the seats in the middle of the plane, which just happen to be the exit rows.

Even even better - the new FR 737's have two emergency exit doors on each side of the plane, so double the number of exit row seats.