No reason why you shouldn't (and plenty of reasons why you should) rate FP4+ or Delta 100 at 50 & process appropriately. If that's not slow enough, add a stop of ND.
What tends to be raised from year to year without resolution is that the 46mm and 70mm roll stocks, which are designed for respooling for 127 and 116/616 are offered in HP5+ rather than FP4+ or Delta 100. The cameras of these formats are often limited in controls and were designed for emulsions using an EI of about 80 rather than 400. Even my fantastic Zeh Goldi folder only stops down to f/22 and has a top speed of 1/125, which tends to rule it out for use with HP5 in sunny conditions unless I use some sort of filtration. If I want to open it up to f/4 or f/2.9, it's just not practical.
On a side note, I wish that an earlier inquiry to Simon about the thickness of the film stock on the 9cm roll had been addressed. I was very tempted to order a roll of FP4 at first for use as a source of 122/125 format film, but in reading about the difficulties one other poster had in doing the same sort of thing due to the rigidity of the stock, and with no change to other items that have been raised, I could only expect the same type of headaches in trying to do this.
Though I haven't ruled out adding more things to my existing order, it unfortunately stands at just one item at present. Had the other film rolls been on FP4 emulsion and all confirmed on thinner stock, my order would certainly be larger than it is.