As I said elsewhere, casting a spell and holding it uses visible components the hobgoblin could react to.
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
Far as I think about it, if one wants to hide a spell, they should pick up the subtle spell-metamagic. Making every caster able to do what is supposed to be a special ability (on par with doubling the range/duration of a spell) cheapens the ability and makes casters even stronger than they already are.
