update
I just finished up the project that I originally posted about, so I wanted to share my experiences in case it may help someone make a decision in the future.
I ended up using the Red Lion Data Station to communicate with the 11 Ultra3000 drives, over the multidropped RS485 connection. I have to admit it has worked pretty much flawlessly. The folks at Red Lion didn't seem to overlook anything when implementing the U3k host protocol. There was one minor issue I encountered where some of the U3k parameters cannot be accessed using gateway blocks in Crimson (you have to use programming instead--no big deal).
I was debating whether to go this route or use the DeviceNet drives, and I have found that there are really only two disadvantages to the Red Lion solution (both of which I was aware of before diving in):
1. Speed. The U3k serial connection is only 38.4k, so that's the bottleneck even though the Red Lion and SLC are on Ethernet. I am transferring about a dozen parameters to each of the 11 drives and it sometimes takes a few seconds to see the changes reflected in the motion. But for this application the changes are usually made during setup so that speed is perfectly acceptable.
2. Lots of I/O needed. I'm still using hardwired PLC I/O to control the drives and select the motion profiles, which requires 8 outputs and 5 inputs for each drive (143 points total). I used Flex I/O on RIO for this since the 32-point modules are available.
In retrospect I think I made the right choice, although DeviceNet would have worked just as well. We just don't have any DeviceNet experience and I was reluctant to go down that road unless it was necessary. I am sure DN would be the way to go if you had higher speed requirements, and many would suggest going CLX but we haven't made that jump yet either.
Thanks for everyone's replies to my original post. I always appreciate the advice I receive on this forum!
-John