Yep, that MOSFET is a beast. I was originally going to do a large bank of 10 or transistors, and then I found that one. With some rough calculations, it wound up being cheaper to use these big MOSFETs instead of smaller ones. Not a big difference, though.
I didn't trust the 0.1 typical case-sink resistance either, and was assuming about 0.25 or so.
Its SOA includes DC operation, and I would be well within that, but it still made me a bit nervous.
To my original question, though, mostly just for curiosity: do multiple heatsinks act as parallel thermal resistances, or is it more complicated than that?