Setting up seanw's hack is proving more challenging than I'd anticipated, mainly because I'm a neophyte with slapd and LDAP in general.
I'm hoping that by posting here I'll get him to come back, or that someone else can build off of what I've done.
My notes can be found at
Ewilen-Notes - Zimbra :: Wiki
If people can feed me tips (here, via PM, in the wiki page or discussion), I'll continue working on the setup and edit my notes into something which will (hopefully) be generally useful.
In the bug for this issue it's been suggested that the version of iCal in Snow Leopard may fix the problem. But I'd like to get this working at least as a stopgap. I'll be demoing Zimbra in a couple weeks, with iCal as one possible client, and it would be great to be able to show it working smoothly.