What I have seen is several posts saying that the mics are too big to stealth with without a hat, which is not a question I have ever asked. I have been stealthing for 14 years, and I have stealthed with larger mics than these. It's not an issue for what I do. I'm a little confused as to why people are trying to help me solve a problem that I do not have.
You said you wanted to buy Schoeps or AKG actives so you can tape musical theater concerts (for which the sound emanates almost entirely from an orchestra pit and/or PA system). You said you don't want to use a hat, and you can stealth with any size mics. That you record musical theater is mostly irrelevant except that (a) there is no value to "onstage" recording like there might be with something like, say, jazz and (b) presumably your seats are always assigned, and probably not always terribly close.
If you don't record onstage, the MK21 is a fool's errand, or at least, certainly not optimal in most situations. That's also my opinion of an omnidirectional mic, unless your theaters you record in are all acoustically perfect.
I guess when I think of musical theater I think of New York, and I am pretty damn sure if I went into a venue in NYC with Schoeps taped to my glasses I would get caught. If you're putting the mics under your shirt or in a pocket or something, then there's really no point having a further discussion, as that's not a valid way to use a high-end microphone to do anything. I disagree that ushers in NYC venues won't notice somebody with microphones on their head (or a video camera, since that's the most common problem they have), but perhaps it is different where you live.
If in fact you can magically stealth with anything of any size, then you may as well buy AKG 414s or better yet, Neumann U89s. They have outstanding sonic characteristics and of course can be switched to whatever pattern you want. Certainly I've heard of people taping rock concerts stealth with such mics; I wouldn't want to go to that trouble, but if your fourteen years of experience makes you so confident, then by all means, go for it.