The ICH is actually able to handle Port Multipliers (the Pro Drive unit is actually a PM chip in a mount that allows 2x 2.5" drives on a single HDD port).
Have you tried running a PM enclosure on the ODD_SATA ports (must assume you have these, given the product you're asking about)?
I ask, as the 2006 - 2008 systems have the ODD_SATA ports as well as an IDE port (what the OEM optical disk/s were attached to). But I'm not sure if there's an issue in the firmware that's preventing PM support or not (wouldn't surprise me, particularly with the 2008 models which I gather you have from your signature, as those have boot limitations on those ports). So other issues such as the lack of PM support isn't unreasonable IMO (first system to use EFI64).
If you've already tried a PM enclosure and it didn't work, you will need to get an eSATA card that supports Port Multiplier based enclosures. 3.0Gb/s versions can be had really cheap (i.e. SIL3132 based, and Silicon Image does make OS X drivers; example), and the newertech 6.0Gb/s version that has PM support isn't horrible either.