Hi BrooklynZoo!
I'm no expert; but, I have one and like it! I have mine on a 20 gallon tank though. I've read here that these skimmers are not much good on larger tanks. (I think 75 gallons was considered too big in the post.)
Before investing the money I did on my large, in the sump, skimmers, I was actually thinking of doing the same thing. That some skimming was better than no skimming and that I would have no problem cleaning the cup daily...
Well... I'm glad I did purchase the appropriate skimmers! I clean the cups every Saturday and Saturday comes way too soon for me!!! <smile>
If you still want to use it, I'm sure you can hang the Prism on the sump wall just like you would on the tank. You may need to extend the intake. Most sumps aren't filled to the top like tanks are... And, clean the cup often...
Some here run skimmerless. Maybe turning your sump into a refugium that will grow the specific plants necessary to do this is an option? Or, making a custom sized DIY skimmer to fit your available space. Plans are available here and at GARF. Seems to me that you could adjust the size once you've thought the plan through...?
My $.02 anyway! <smile> Good luck!
Linda