I'm not significantly worried about the CAT ratings, I'm more worried that I am misunderstanding them, or other aspects.
The Pintek CP-3308R for example does state a rating of 1.5kV CAT II. Going by the table at
http://www.ni.com/white-paper/5019/en/ I would expect it can handle transients of at least 8kV (maybe more?), and so it seems as if it would provide adequate transient protection (although I suspect any transients would be attenuated\absorbed by the SMPS input protection circuitry anyway). Without the datasheet saying so, though, I do not know for sure. Do you know?
I don't see a big reason to worry about using a 100:1 probe on any of my oscilloscopes and probing 400V or so in an SMPS. The divided voltage should be quite low, as long as I use DC coupling as
Hydron states. (I already read about that sneaky fact) Therefore, I don't really see a problem in terms of input voltage with what I want to do, provided the probe is good quality and does what it says it does. The Pintek CP-3308R being good for 2kV up to 2MHz seems like it would be sufficient.
I suppose I just want to check that I'm not making a mistake in my reasoning\understanding.