~~That isn't how I interpret the ready action.~~ And yeah, there should be a save of some sort even if it isn't RAW, but I think I'd allow it out at my table, it's creative and fun.

Edit: I reread the rule, there is language for precasting the spell. I stil thinkl it'd be more fun to find a way to hide it, like a stealth check or similar.

Either way, the fact that casting a spell is an action isn't really a problem here