This is a good tactic, using a Quickened spell to trick that ready action. However, if there was no Featherfall spell the caster would normally have to have quicken spell and burn a 4th level spell to make it work. Since that's such a disparity in levels, I would make Featherfall an exception (it already is really) and rule that it is cast so quickly that it doesn't trigger the "if he casts a spell . . ." ready action. To go with this, I certainly wouldn't allow a cantrip that is cast as a free action to do it either. I'd give the player back his research costs and tell him you made a mistake -- if he researched it just for this purpose.