Tom, I use a Hasselblad 30mm with a Phase One P25 back and it works fine as a fisheye, and in emergencies you can indeed straighten out the image and use the lens as an ultra-wide rectilinear.
The image circle of the P25 is about 61mm so it effectively edits out the very lowest quality from the corners of the 30mm, if you're reproducing the straightened out shot in a brochure at A5 or smaller it's fine. At A4 in my opinion it's at the limits, and at anything above A4 the lack of resolution with a straightened out shot starts to show.
I disagree that it's a particularly rare lens, I've got an old C series 30mm, so they've been around for years. And Hasselblad always did a good job of marketing and promoting the 30mm, which means you'll see them turn up on Ebay every few weeks.
Regarding other wide angle options with a P25 and the V series, you've two main choices. I regularly use a 38mm 903 SWC for interiors and it works beautifully with the P25, minimal colour casts which are easily removed with the Capture One software, superb close range as well as infinity performance, virtually zero distortion, and sharpness which is still breathtaking for a 50 year old lens design.
The other V series wide-angle option if you're chasing high quality with digital is the relatively new and wallet-busting 40mm IF CFE. I was never very impressed with the previous Zeiss 40mm lenses, but the 40mm IF is outstanding, which at the price it should be!
It's every bit as sharp as the 903, maybe even sharper. The only problem, besides the price, is the distortion. Over the years I've used pretty much every lens in the Hassselblad line-up and without a doubt this has the worst distortion. However, it's possibly the sharpest retrofocus, medium-format, extreme wide-angle lens there is. It works on my 203FE as well as 500 series cameras, and it gives a reasonable range of movements on my Flexbody, so for some photographers it's a realistic choice.