1. You should figure out how to sit the subject in bright daylight, while using screens and/or reflectors to take some of the intensity out of the harsh sun and also fill in the shadows a bit; this will reduce your exposure times (the bright sun, that is) while working to improve the contrast and tonal range. I would consider lighting to be at least as difficult as the camera building; perhaps the biggest single challenge.
2. You'll need some portrait-appropriate backdrop or background, keeping in mind that whatever is behind the subject will be equally sharp as the subject themselves. And, keeping in mind point #1 about daylight exposures, this means rigging up some backdrop out of doors.
3. Wind and/or breezes may come in to play, perhaps moving the subject's hair a bit; this is just part of the fun of outdoor pinhole portraits.
4. The shorter the focal length, the smaller the focal ratio; you may thus want to consider an ultra-wide angle, short f.l. 4x5 design, whereby with sub-f150 apertures you'd reduce the exposure time even more. Shoot whole-body portraits with this type of wide-angle camera, within an existing environment (i.e. it'd be too hard to make such a wide backdrop, just work with the surrounding environment).
5. Fuji Acros for B/W has much less reciprocity failure, giving you reduced exposure times for pinhole.