Hey Matt,
I've done some NULF work, but I don't have the patience for too many 90-minute exposures. Personally, I'd use a faster film (ideally TMY) and open up a stop or 2. Looking at the image you posted, I think you could have gotten all the depth of field you needed with simple movements and f/16. If that lower left foreground was a little soft, it would make no difference IMO. But, if you like 90 minutes of Zen, disregard the above. Here's one shot on 14x17 HP5, 24" Artar, about 25 minutes @ f/22(?).