Being all up in union at my carrier and well versed on our negotiations, I can tell you that the MEC and the Neg committee have asked for the JSA (jet service agreement) with our mainline partner several times even agreeing to signing NDA's and we were told every time to pound sand. They are extremely secretive about it.
In their defense (I know, I feel dirty for saying it too), if not legally compelled to disclose, why would you. Basically you can claim anything you want in the name of "well, based on our JSA, we can't do xyz...". It would also open them up to massive criticism on how they negotiate their contracts both from the pilots, and other carriers. Maybe my company was able to negotiate getting paid for canceled flights and regional B owes the mainline when flights are canceled. I'm sure they would love to know that.