Since getting nice sized sd cards, while it is something of a pain to downsize videos so they take up little space, even though I'm using the video playing more than I ever foresaw I am beginning to see USB host as a much smaller deal - and I'm the guy who carries around a 250GB HD on a regular basis.
Technically, nearly anything is possible with the EXT port. That said, even most industrious devs don't necessarily have access to or skills necessary for tinkering with it to produce such a thing.
I'd say roughly even odds on EXT addon and SDIO.