I'm interested in this, too.
Synchronization options now are like a drawer full of spaghetti.

Not a high-content reply, but it's neccessary to make one to get the Subscribe button!

using syncevolution on n900 most recent version you can configure dav backends. I am using this with sabredav and I have the entire contents from the addressbook.db on the n900 in mysql records as vcf files with all of the X- type fields preserved.

don't really have a frontend for PC editing but you could phpmyadmin, mysqladmin, or maybe whip up something fancy and web based with a php vcard parser

EDIT:revisiting this, I found emclient http://www.emclient.com/ has a nice carddav client and seems to support most of the maemo fields