Yes they will be used in an overhead environment..... instead of moving the tank from the butt plate to added D rings I'm going to try something similar to what eternaljonah did in this post:
http://www.scubaboard.com/forums/si...nting-al80s-how-do-you-trim-your-tanks-3.html
Eternaljonah's solution will significantly reduce the 'play' that the sidemount tanks have to adjust/move through confined environments. This may pose just as much hazard as putting weights on the cam-bands.
Sidemount cylinder attachment is supposed to enable substantial movement of the cylinders to permit the diver to move, flexibly, through an environment. This 'double-D-ring' arrangement seems to be the antithesis of that...
Add to that the increased task-loading of having to unclip two bolt-snaps each time you wish to detach/re-attach the cylinders; for instance when donning/doffing cylinders, when partially detaching cylinders to push ahead, if entrapped in a restriction, or in an emergency when the cylinder/s may need to be swiftly jettisoned... Add to that future considerations when having to handle stage/deco cylinders...
I can't speak for other instructors, but for me... that solution seems likely to cause such a significant impediment to diver function that it'd make it near-impossible to meet the performance standards integral in my overhead environment courses, especially when considering 'timely' operation, single-handed function and the addition of other task loading/emergency management demands...
As a rule-of-thumb: When a solution to a problem causes other problems, which require further solutions... you've probably chosen an ineffective solution..
Ignore the butt-plate altogether... it's not
meant for Ali (buoyant) cylinders. Use multiple, or sliding/adjustable, D-rings to enable cylinder trim as cylinder buoyancy varies over the dive. This is how experienced sidemount instructors teach. They do so for a reason - it works. Avoid short-cutting and compromise through equipment solutions to skills problems. Maintaining cylinder trim with Ali tanks is a
skills issue.