Well, us Canoners are out of luck, but in MF world most digital backs go down to iso25, (maybe iso12).
Or on the film-front, there's Efke RB25, über-fine grained iso 25 black and white film, that you can pull down to 12 or 6 (i've got a few rolls, i'll let you know how they turn out), not sure if Velvia 50 can be pulled down to 25.

But back to digital, most cameras (according to dxo testing) don't actually do anything to the sensor to get it sensitive down to 25 or 50, they only throw away half the info for a longer exposure time. So you get longer exposures without the NDs, but no increase in quality or noise...