Plugin to hack in custom fields when using XMLRPC?

Is there any way to use custom fields when using an external blog editor? I suspect XMLRPC has no provisions for the custom data fields, so….

I have an idea for how to sneak them in using a plugin, but unfortunately, I have no idea how to work the custom field part.

The idea is to use a system similar to what Scott Reilly’s “Hide or Cut Post Text” plugin does. Simply use tags like:

<!--customfield="var:value"-->

… in the body text, hook into an action like edit_post and then work some magic to add the custom fields. That last part is where I’m stuck.

Is this even a workable idea? If so, could someone point me to some code that might help?