That's good thinkin' -- just flipping a normal in-line reactor with a diffuser in it upside-down (i.e. water flowing up towards the tank) you might even be able to control the amount of "micro-bubbles" you get by tilting the reactor a bit, or having it vertical, depending on the thickness of the pvc/et cetera you used to build it.
I think it would be critical to have it enter the reactor just before it went up into the tank though, to ensure that some of the bubbles stuck around. Seems like it would work best with something like a lily pipe as opposed to spray-bars, et al.
That still leaves the issue of diffuser maintainence though...
You could build something along this line, with a chamber wide enough for a good-sized diffuser disk, and a removable lid of some sort with valves to shut when you need to remove the lid and clean the diffuser.
