Orcus on a bad day
Dammit, being reasonable. What type of warped internet debate is that?That is something to think about, but it isn't much more of an issue than trying to do the same thing with other actions--something that isn't allowed, but often you have to come up with some way to handle it when people want to do it in the fiction. I'd probably just make them roll initiative normally and not be able to release the spell until their turn comes up. If they were expecting a battle, I might give them advantage (as well as to anyone in that boat).
The goal isn't to provide a way to cheat the system between combats, but to not be overly harsh inside combat. Another thing that would make sense out of combat would be to require Constitution checks (or Constitution saves) to keep holding the spell for longer periods of time, since it strikes me that this is a bigger deal (you're kind of stuck mid-cast) than the normal "Concentration" mechanic, which doesn't mean you're actually concentrating at all (more like you've got a magical effect taking up a certain amount of your magical capacity in a barely conscious way).
Okay, I see your point. And you're right, people will try to Ready anything. As you as you have reasonable players (and I find they are the most fun to run with), it works out fine.