I can't say that I've ever experienced 'subspace" (or if I did I didn't recognize it as such) Of course, I've always thought of myself as more of a sensation player/bondage bottom than a true sub, slave or masochist so that may enter into it.

I certainly have experienced excitement, trepidation, adrenaline rush, intense sexual arousal and perhaps even a certain amount of euphoria while engaging in BDSM play activities ranging from very painful to intensely pleasurable. However, I'm not sure I could characterize any of that as an "altered state of consciousness".

Motivations and state of mind during BDSM play and how that effects behaviour and physiological reactions is a fascinating topic.
Thanks for coming up with it.