Just want to update everyone with my current thoughts about this. The option I'm currently leaning towards for 2012 will be to use a Slave unit to run all the PixelNet stuff (and thus add just a small sync signal to the network), and then run the E1.31 stuff directly from the scheduler running on the PC (either LSP, if that one works better this year, or xLights). Assuming LSP and/or xLights supports sending out the Slave sync signal in conjunction with E1.31, I think this provides a nice trade-off... yes, it still requires a PC, but hopefully that PC won't need to be as powerful, since it really is only directly controlling the E1.31 devices/channels, and not the PixelNet stuff.
Perhaps this is more in line with what RJ was thinking when creating the Slave unit, anyway.
Another option that I've come up with, although I don't have the technical skills to pull this off myself, would be the creation of some sort of "PixelNet to E1.31" converter. One issue with this might be the cost of such a converter, and I'm not sure it would have wide applicability.