Could you format the SD card on a modern Mac using Apple Partition Map with the HFS+ filesystem, and then simply take it to an older Mac and reformat the existing partition as HFS standard via Finder's "Format..." command? That shouldn't mess up the alignment of the partitions, should it?

I'm tempted to try this myself (though I first need to get around to setting up a SCSI2SD for an SE/30 I've been working on. Slowly.)

c