I have a post that shows a chart the explains the relationship between different airline regional models.
Regional Airline Models
It would be difficult to turn a regional into a Codeshare. They would need fire up their own marketing and ticketing department, and assume a whole lot of new risk. It could be done, but their would definitely be a steep learning curve.
Fee for Departure and Pro-Rate agreements are well covered under most regional scope agreements.
To really Code-share you would need to operate as a fully fledged independent carrier without a mainline flag or livery.
SkyWest has been the closest to being successful with their Airtran partnership, but even that was more of a self branded pro-rate agreement. It would be tough also because with a Code-Share agreement you lose alot of the "protections" afforded in a Fee for Departure or Pro-Rate agreement, and could easily be "hosed" by your "partners" if they withdraw their networks.
Only way I could see it work would be in Alaska/Horizon's case where you have a very established route structure and network and could be
ok if you suddenly lost much of your codeshare. Otherwise it is very risky for the Regional.