If it ain't broke, don't fix it
Vantec's Nextar enclosures are the brand I use. I know they have an internal SATA controller, but I thought it was only used for USB connectivity and that eSATA was merely a pass-through. But apparently the internal controller is used for eSATA too.
At this point my best guess is that the Pal's SATA controller won't negotiate a SATA III drive down to SATA II, but the enclosure's controller will, allowing your drive to work with the Pal. At any rate, we now know an external enclosure will let at least one additional drive model work with the Pal.