Sounds like a nifty idea......however,
I see a few possible problems
1. sanitation, those valve blocks were not designed to account for bacterial growth, this means cracks and crevises for critters to hide in as well as whatever seals used in the blocks may be permiable and give bacteria a place to live.
2. the past use of those blocks, most pneumatic valves require the compressed air to carry a small amount of oil mixed with it to lubricate solonoids, seals, tools ect, that oil will be embedded in the seals and be tough to clean out. so if you were to run beer in them you would probably pick up off flavors
3 temp rating on the valve system, unless you are planning to use this for chiled wort and water only, I doubt those valves are rated for running high
temp wort and water through them, again hot wort would definatly pick up any air lubricaion oil reside left behind. furthermore should you manage to clean all the oil out of the valve body "hard since its a very thin film" the valves would break down eventually from lack of oil.
4. hose diameter, unless those are big valves your hose id would be pretty small and you wouldn't get very fast flow AND most compressed pneumatic systems leak a little "especially at the valves"
final thoughts " i really like the idea of pneumatically moving beer around, you would need properly rated valves to do it. should your valves fit the criteria i named above give it a try I'd like to know how it works
5. my advice, sell those valves on ebay or wherever and use the cash for a march pump