I'm not familiar with your gear and not quite sure what you are up to, but what kind of resolution are you trying to pass? I know that there are products in the Meridian line that have RS232 and volume control like you need. The 568 might be an option for 24/96 on its own, or the 518 into your current DAC for 24/48 tops.
DAC with RS232
I am using a 2 channel Bel Canto DAC in conjunction with a 4 channel Trinnov ST2-Pro processor in a 6 channel system. The 2 channel processes my surround channel. I synch up volume control through iRule. Problem is the Bel Canto is IR and sometimes misses a command, and my volumes get out of synch. Ideally I would use a 2 channel DAC with RS232. I know these exists in teh very high end (EMM Labs, MSB). Is anyone aware of a more affordable DAC (say $1000 used tops) with RS232 (and obvioulsy volume control). It needs to have 1db volume increment steps to be able to synch up with the Trinnov. I have not found any myself.