You could use custom SQL SELECT ... FROM my_table ORDER BY RAND() LIMIT (N)
He Fred, that's a slick idea. I was mulling over the best way to do this -- in the course of programming an online game, I needed to pick n contestants from a pool of arbitrary size, then narrow that to n/2 contestants, then n/4, finally ending up with just one.
Adding and indexing a random number beforehand sounds like a good way to go. Thanks for the thought!