That thing rocks! I need something like that, but for a somewhat different reason. I bought my GR with a mulch kit on the '54. Kubota's mulch kit sucks. Or blows... Really nasty clumping and striping.
I ended up taking it off and going to Gator blades. Not really a mulch cut, but not too bad. Problem is the discharge hurls grass, and as you've pointed out, there are lots of places where that isn't good. (Mowing along a raised bed, I end up looking like I was wearing grass-colored cammo)
For now I've partially blocked the discharge with some aluminum angle stock. That sort of splits the difference - like my old Deere mulcher that had an adjustable discharge and worked very well. Just enough spit to not clump.
I'm going to try to jigger up something like your gadget. Without your skills it is sure to be ugly...
Thanks for the great idea!
Z.