The cost of such adapters would be at least the same price as a BB to fit the crank arms. What you're after is a rather fancy bit of kit. Also the whole point of a splined crank such as Octalink or ISIS is to reduce the flex in the BB. So why would you WANT to install an adapter that allows you to use a splined crank arm set with a square taper BB?

So basically you've come up with a solution to a question that no one has ever asked or that anyone SHOULD ask. Don't feel bad though. I've done the same thing far more than once...