Too bad. I believed that the rfx routing was really flexible. I also dreamed to use an external fx rack for reverb, but ok, it's just a dream.
Why is it a dream? Your idea for use was one of the reasons they developed the rfx card. I believe if you delve a little deeper into the manual you will find that there are ways to use the rfx as an external fx unit for anything.
I use it to insane stuff with my bass. Please read page 46 of the RFX manual.
"Processing External Inputs"
If you have any of the input expansions you can even route more than one stereo pair through it.
Its quite flexible. You can even route the signal through the insane EMU filters which is described on page 47.
Hope this helps