Is it ok to introduce caged virgin queens directly into a hive that has been queenless for 24 hours or so, or is it better if they are mated?
Mated is better.
You'll also have better luck introducing virgins into nucs (small amount of bees).
another trick I know works well is if you need quick acceptance of a virgin queen put honey on her, not enough to drown her but get a Q-tip and swab her whole back end good and they will should take her in fast, seen it done a few times, they will clean her in no time and except her, Dwight Porter showed me this trick. of course you will have to handle her to do this so if you are know sure you want to do this then introduce in a nuc.
Funny, Alley and others used to just dunk them in honey. Seems like a bad idea doesn't it?
http://www.bushfarms.com/beesalleymethod.htm#introducing_virgin_queens
http://www.bushfarms.com/beesdoolittle.htm#CHAPTER17
http://www.bushfarms.com/beesdoolittle.htm#CHAPTER16
http://www.bushfarms.com/beesbetterqueens.htm#Introducing%20Queens
http://www.bushfarms.com/beesqueenrearingsimplified.htm#c27