It will depend on 1) whether or not there is a mechanical limitation (bellows, draw, lensboard size, etc.); 2) what you want to do with the resulting combination; and 3) exactly which Velostigmat you have (the name was used for a considerable number of designs).
(3) seems to be easy---from the picture, the aperture is 4.5 and it looks to have the variable-softness ring on the front, which would make it a Velostigmat Series II soft-focus lens.
Having put a non-fuzzulator-equipped 9-1/2 inch Series II on a bellows for 2-1/4 format, I can tell you that enlargement will exaggerate the effects of a given amount of softness in the negative. However, cropping the center two inches out of a field that is at least five inches in diameter has the opposite effect, once you are down to f/6.3 or so. By f/8, the effect is pretty much that of a not-very-good conventional lens, at least for the one that I have. (Wide open, there is precious little depth of field if you are going to 4X enlargment of the negative.)
You are talking about a rather longish lens for portraits in this format, and the negatives are likely to be fairly soft due to both the lens design and the scattered light from the unused coverage.
I, for one, would be interested in knowing how this works out, if you go ahead with it---I always wanted to try one of the variable-softness lenses, but never found one at a price consistent with the idea of doing the experiment.
