As I poke around for more drive enclosures, I found a link to this pretty nice looking one from IcyDock.

The horribly named IcyDock MB561S-4S is a 4-bay SATA enclosure, with eSATA external interface. There are 4 eSATA ports on the chassis, which I'm assuming means there'd have to be 4 SATA cables going from the drive to the host computer. Not the cleanest configuration, in terms of cabling, by far. The drive doesn't support RAID by itself, but if we're using ZFS we don't need a built-in RAID controller anyway.

It's a pretty nice screwless hot-swappable design. This is much better than the qBOX-F Firewire enclosure I looked at earlier.

IcyDock 4-drive SATA enclosure

The IcyDock MB561S-4S seems to be in the $200-250 range at Newegg and other resellers, but it's only $149 at Buy.com (after a rebate). During April 2008, there's a $199 mail-in rebate on this enclosure. See the reviews from NewEgg.com.

My only additional cost would be to add a 4-port eSATA card to PowerMac G5, for around $140, plus some cables. But that's a cost I'd have to factor in for most all of these eSATA enclosures (even a 2-port SATA card is around $100 or so).