Yeah, subchannel usage would be great, but I think it's impossible. At least for the Q-Subchannel (because it's used for navigating, I think). Maybe you can use the subchannels R-W. But that doesn't give much space to store data in it.

Also, subchannels aren't really "error-protected".
Example: Read a disc with CloneCD, subs reading enabled, twice.
And then compare the SUB-files with fc /b or any other tool.
There *ALWAYS* will be differences, no matter, how often you read ...