I actually tried something similar recently, roll of Provia 100f. I used Diafine as a first dev, exposed to light, then developed in C-41. What came out the far end were really dense slides, they looked about 2 or 3 stops underexposed. The colours were surprisingly all right (bit of a blueish cast) and they scanned reasonably well.

I think the general consensus is that the first B&W dev has to be pretty active. Diafine just didn't do the trick, I reckon its nominal speed for C-41 or E-6 is probably a couple of stops slower for a start. I'm going to bracket the next roll +1 +2 and maybe try and develop half of it twice in diafine and half the once and see what happens.

Some results are here: