I think so.  Assign Windows L/R to RME AN 1/2 (middle row in Totalmix), and then use Totalmix to have the L/R appear at your SPDIF Out or one of your unused ADAT Outs (say ADAT 1/2).  Enable Loopack on that SPDIF/ADAT pair, and that will feed your DSP input. Then have the DSP send its 4 processed channels to other unused RME ADAT channels (ADAT 3/4/5/6), and use Totalmix to re-map those to Analog Outputs 1/2/3/4 and off to your monoblock front stage amps.

Then Windows *should* use RME AN 3/4/5/6 (middle row in Totalmix) for the remaining Center/Sub/Surround channels if "Interleaved" is enabled - simply re-map those with Totalmix to spit out Analog Outputs 5/6/7/8 and off to your center/sub/surround amp(s).  The surrounds shouldn't receive any signal for regular 2-Channel sources.

Assuming the FF400 does have 8 Analog Outs - this *might* work...  Just checked - The FF400 only has 6 Balanced Outputs - but you could likely use the Headphones Out to feed your Surrounds in a pinch!  Or use an external 2-Channel SPDIF DAC for the additional 2 analog outs.

You might also get some DSP Delay from the Front Speakers referenced to the rest of the "un processed" surround/center/sub signals

Give it a shot and report back.  Sounds like a fun project!