Well . .. um . . . I'm doing that. With the same character's POV. It's my latest, my YA, that hopefully my agent will be shopping soon.
My use for it is similar to yours actually. My book is a re-imagining of Shakespeare, so for the actual plot everything is told in 3rd person past, but then I interject what I consider soliloquies, told first person present. The idea is that my main character is telling his own story in third person, but every once and awhile decides to talk in the moment to his readers.
I guess my point is . . . it's doable (lol, I hope it's doable, I mean so far out of all the problems I've had to fix, the POV switch hasn't ever been mentioned), but make sure you have a concrete reason for doing it. Oh, and something else I've learned, try not to disrupt the action too much with the first person. You don't want your story to suddenly stall. Spend more time moving the story forward, than the introspection. Otherwise your readers are going to get uber annoyed.