I don't know of any that'll update on the fly, partly because the ram drive created doesn't give enough space to download and use most defitions.
You might be able to create a BartPE disc, then use a tool like Flashboot to put it on a usb key, then update to that. You obviously have to have a machine capable of booting to usb to use it, but I can't think of another solution.
I'm very curious to see what anyone else suggests.