It appears it was a behavior change on the server end; 9.0 went B&D on the type system. I have no clue where in their docs, if anywhere, this is discussed, but I ended up doing mailing-list archaeology from two-plus years back, found that option, applied it, and voilà.

So short version: This works in 5, assuming, of course, that you set up the enum type and column definition explicitly, but you have to use the secret handshake in the JDBC connection URL. Maybe a candidate for inclusion in manual section 19.6.