Craig,
That approach has potential (depending on the ability of the reactor to keep the Purigen beads out of the aquarium and that the mesh screening does not become clogged). Be sure that you have sufficient pleated filter area for the mechanical filter and that it filters down to about 30 u. My mechanical filter provides 60 sq ft, I have 10 adult discus and my cleaning schedule is monthly. You can scale your system accordingly. For example if you have 5 adult discus and are willing to service the filter twice a month, you only need 15 sq ft of pleated filter. My estimates here are conservative.
I would also suggest adding a second valve to the input of the reactor, and also other valves that will let you service the both devices without spilling allot of water.
Not sure what you mean by "placed on the return side of a sump."
Don't forget the other considerations called out in my posts.
By all means keep us posted as to your progress.
Thanks for your input.
Paul