Multi-select from Cascading Select Customfield

This is probably a duplicate of http://jira.atlassian.com/browse/JRA-7257, but that issue has not been addressed in quite some time, so I thought it wouldn't hurt to re-up it as a request for 3.6.x.

The Cascading Select customfield is extremely useful. It would be even better if a Multi-select version of the custom field existed, that allowed multiple values to be selected from either of the Cascading Select menus. Is this:

a) Possible without the creation of another custom field [if so, is there some example that I was unable to find in a web search]?
b) Something that could be added as a custom field type?

Thanks for any responses.