I looked around a couple weeks back to see if anyone has gotten the openRD to work with a SIL3726 port expander and I didn't find anything conclusive. So I'm here to say it "works" with the default 2.6.22 kernel that came on the device. By that, I mean it sees hard drives on the first 4 ports. The 5th port is somewhat of a mystery as the device seems ok and comes online but the mvSata driver never discovers it. For future reference I'm using the Sans Digital TR5M-B. I plan on upgrading the kernel and turning the mvSata debug logging on to see if I can discover why it doesn't work with the 5th port, as nothing obvious in the code seems to limit it to 4 devices.
So, now my big disappointment is the fact that the sans digital consumes 12 watts(!) totally idle without any drives plugged in. That power seems to be broken down as ~1 watt for the large cooling fan, 6 watts for the multiplier board, of which the primary thing on the board seems to be the SIL (which is getting fairly warm). The power supply with a little 1 watt cooling fan consumes the remaining 5 watts plugged and running with everything disconnected. Its a 250W power supply and I'm running the Seagate LP drives which seem to consume about 5 watts "idle" and ~15 watts at spin up. I might try switching out a low power supply in the near future. Nothing in the open Sil documentation would seem to indicate that its power usage is that bad.
Also, as hdparm doesn't seem to work, I hacked up a little utility to force a hard drive to spin down when using the marvell driver. This drops the power usage from ~5watts per drive to ~1 watt. I will put the code to do that in another posting.