Mistress Sonata asked this of my Sir Conundrum post:

I wonder if you could maintain the subspace but have the sub say "Master/Mistress, may I request a sidebar" which is like a safeword and will always be granted, like a "light Red" where you can discuss something in scene. Say you ask your sub to do something you haven't discussed prior and your sub wants to get it across to you that while they're still in obeisance to you and don't want to stop the scene, they don't want to do whatever it is you're asking of them. Then you acknowledge and go back to the scene and do something else (rather than stop the scene entirely). Thoughts?

And of course I am more than happy to respond! The short version of my feeling is, though, "not really".
The problem is that subspace - any fixed or fixated mental state, really -  is going to be past the point of rational decision-making about the situation they're in. A lot of subs, when they're floating in sub-space, will have serious problems trying to do so. Some won't be able to form words at all; some won't be able to remember the safeword. Even if that isn't true... Someone in subspace, as a rule, won't want to "break character" - they're strongly disinclined, by the nature of what they're doing, from saying "No, I can't do that" and displeasing their top. Or, hell, maybe they're gagged.

Even if the sub is tracking straight enough and wants to stop and say "No please, Master, I don't want to stop but I'm not okay doing that..." (a difficult enough thing at the best of times, let alone when in an altered state!), what if the Dom/me is in their own head space and doesn't respond? Subs and slaves aren't the only ones who can get deeply into the scene. A top can get "in the zone" and miss it - or not recognize it - or just forget.

If all that works - the sub can remember it well enough and get the question out, the top can respond correctly - subspace can be fragile. The more you interrupt it - by essentially interjecting an entire conversation into the situation - the less likely it is for subspace to survive.

So... Can it work? Yes. But... Only if the scene itself is already essentially outside of subspace - if it's a person playing at subspace, rather than someone actually totally in the mindset.

A better alternative is just the old standby of having multiple words. The example I've always heard as most common is red/yellow/green: red for "Oh my god no we need to stop RIGHT FUCKING NOW"; yellow for essentially the situation you describe above, a "this is pushing me out of my happy place, we need to get back on track"; and green for "Yup, all systems go." It could also be "bronze/silver/gold" or just about anything - "asparagus/rutabaga/mushroom". Something you can remember and that won't otherwise come up in your conversation.

I will also say, in the way you have it phrased there, I think there's a serious failing going on - on the side of the Dom/me. Why, when you're deep into a serious scene or situation, would you pull out something potentially threatening or dangerous that you haven't already spoken about? To me, that's a serious breach of trust on the part of the Dom/me. That's a failing in putting them in the situation in the first place... And there's really no good way for the sub to respond. Either they're broken out of subspace enough to respond rationally, or their trust is being abused in a way which is totally unfair to them.

And of course the continuation of the thought is that, in an ideal world, safewords shouldn't be necessary. Not that they aren't necessary; not that they can't be useful and important; not that they aren't an exceptionally good tool for people still feeling their way through their relationship... But it's a top's responsibility to know their sub's state - to take care of them, to keep them safe. It's the top's responsibility to not throw their sub into that type of situation.

The moral? Don't spring surprises on them when they aren't prepared to handle it. It will only really work well in a tiny minority of relationships.