"Might could" is part of my regional dialect.

As a response to "are you going to do it" I would respond "might could" or "I might could." The beginning of the originally listed response, "I'm not sure," isn't necessary because "might could" implies uncertainty about the speakers involvement.

I like another comment on here where "might could" is expanded to "it might be the case that I could." This is how I've understood and used the phrase.