If you can get hold of some Fuji Acros, also soon a new version called Fuji Acros 11 will be released, the beauty of this film is no reciprocity failure up to 2 mins, there after double the time, Kodak Tmax x 100 iso is another good choice with low reciprocity,
I also like and used the folliwing, Ilford Hp5, FP 4 and Trix, these will give much longer times than the the others due to reciprocity failure, that has to be added to the metered and calculated times, so if your after longer times for motion blur such as sea scapes, cloud movement, try the 400 iso films, HP5 is my favourite
Here in the desert southwest of the USA, a 400 speed film would be difficult to use in that bright, sunlit exposures (@f/138) would be less than 1 sec; maybe 1 sec. Personally, I don't think I could adequately control a pinhole exposure for that short duration. FWIW, if needed, there are other medium speed films that don't have the extreme reciprocity failure of, say, Foma 100.
I use HP5 with my Super Graphic hand held at 1/100 (or 1/125) @ f22 sometimes 1/200 (or 1/250). at f138 that would be an exposure of 1/4 or 1/8. So on a sunny day HP5 isn't practical without a shutter.
On the other hand, if it gets the result you want...
Will anyone care, as they admire your work on the walls of the Tate, that you used a shutter or not?
Martin, I've tried using filters with a pinhole and even with the most rigorous cleaning, invisible specks of dust can sometimes be recorded as blotches. I've even tried mounting the filter inside. Do you have a secret way to avoid this?
Perhaps my dust fell on the filter during exposure. I've never had exposures that were less than a few seconds. Or perhaps my enthusiastic cleaning generated static. We shall never know. Best of luck with yours.
You might try a falling-plate shutter. Just a flat plate with a slot in it that (obviously) falls. The length of the slot determines the exposure. I saw this used for a reconstruction of Muybridge's galloping horse experiment by the late Terry King in 2004, on Ham Polo Ground.
Terry set up a row of twelve volunteered 10x8 cameras to capture a very handsome pony galloping past. The original experiment had had strings stretched across the horses' path which they broke as they passed, thus tripping Muybridge's shutters. Modern horses refused to do this, as they mistook the strings for an electric fence, so another strategy was devised using a paint roller. (QV)
The owners of the 10x8 cameras were naturally reluctant to have a galloping horse and a piece of string attached to their Copals and Compurs, so the shutters were opened before each run and a specially-made falling shutter placed in front of each one.
Altogether, a very interesting day. Everything from Gandolfi to Ebony.