Yep, we use Gerber trainers (and some similiar that are another brand, can't recall but same exact thing) (the ones that are like undies with a padded center, not the waterproof ones)...if he really pees, they do no more good then if he had undies on, but we often have accidents here where he'll start to pee, realize he's doing it and stop and go to the potty...so having something that'll hold that little bit is nice for keeping carpets & furniture dry....pants usually have to be changed, but we've used wool longies over them the same way you would a prefold too.... if you already have some then use them. Otherwise we got regular sweatpants a size too big from walmarts; not waterproof at all but easy for him to pull up and down.
My DS doesn't care less about the character undies, we let him pick some, (cars & elmo); but he treats them exactly the same as the plain ones. But I know some kids do much better with that type of thing.
I don't think FB or BG's without an insert would keep your furniture dry at all. Might contain poo messes, but most trainers & undies will too in a 2.5 yr old, as long as he doesn't have diarhea (SP?). I could be wrong though.