Hi Mark
Do me a favour mate and reach back behind you with either hand and touch your spine.. how far can you slide your hand down your spine?, I'll bet even with your injury you'll still be capable of reaching to at least your second vertebrae and as Lanny said, if you can do that then you can reach your valves.. honest guvnor!
When I got my twinset I couldn't even get close to my valves.. I had to get my buddy to turn my air back on after an instructor had turned off my primary during a course... very embarrasing
Now at that time I thought I'd never reach, I thought the problem was physical and that I just wasn't flexible enough, I knew that I couldn't progress my training until I could reach my valves and so decided to sell my perfectly good Oceanic Aedura and then had a made to measure suit from Otter.. result... I can shutdown my primary in seconds and can reach all three valves with no problem at all, infact during dives I've trained myself to keep reaching back and manipulating all three valves while repeating to myself "It's open, I can close it, It's open, I can close it, It's open, I can close it" This also helps to develop the muscle memory so that I can hit the valves toot sweet if I need to.
Selling a perfectly good suit may seem a bit rash but if that suit proves to be restrictive then it's a much better option investing in a new suit then it is completely bastardising a perfectly good hogarthian rig to make up for problems with a drysuit.
Once I sold my Aedura it cost me an extra £200 to get my new suit.. I imagine thats not a lot more than buying valve protectors, custom hoses etc and the end result is my rig is the correct way around...
The point of this post is don't give up Mark, I'll just about guarantee that with the correct amount of flexibility in your suit and your harness set up properly then you WILL reach your valves....
Good luck mate
Dave
p.s. I do have a link to a dive fitness website with lots of pictures of shoulder stretching exercises because I believed I needed them.. but once I got everything configured as it should be there was no need for the excercises.. for me anyway.. when I get home I'll post a link