As acold saltwater,primarily open water diver with back problems who is considering sidemounting as a way to ease out of twin backmounted 100 hp's and who had thought perhaps out of backmounting to ease up the pain, I also recently asked almost the same question in a separate thread.
I have not yet heard and cannot at this time see any reason to sidemount a single cylinder.I can however see a few reasons why when looking at planned gas use in the 70-75 minus cubic foot range depending on the cylinder and its pressure to allow adequate redundancy of gas that anyone adverse to carrying extra tanks for no more reason than balancing themselves and or adding more ballast weight in the case of using alu 80's , not to mention most times using an alu 40 I have a richer mix than my back gas aboard. (I have considered sidemounting a couple of these alu 40's for shorter dives but find 80 cu feet limiting when compared to a pumped up lp95.I can see maybe using lp 72's although I do not own any.)
While I had hoped to free myself from my backplate and wing.I see that one must use different systems in different situations where each system's merits and strength's are best for the individual and or team.
Regretfully, i will stick with a back mounted Faber lp 95 and 40 cu ft for now and whenever i find a suitable sidemount BCD and the time to train myself to competency for either broken up 100 hp's twins or paired lp 95s and a deco bottle or 2 i will consider throwing cash at that specific task for sidemounting.
Estaff. Here are three ways to deal with your situation that have absolutely nothing to do with sidemounting a single cylinder as it sounds like a PITA to me although I have never done it with anything larger than an alu 40 for inspecting a vessels hull.
Stick the 100 hp back on a backplate and wing and sling your 40 when diving this dive plan , take both 100 hp's and carry a pile more stuff for no reason at all beyond trim or add to your personal tank farm to get yourself a couple smaller matched tanks for these occasions is what i hear.
Bill