Customizing biped

I don't think anyone has submitted this particular question, so please tell me if there is a similar thread..

what I'm trying to do is customize the biped by changing some of the bone(s) link info, but I couldn't find on how to do this. this is the detailed task I wish to perform:

In each Clavicle bone of biped, you wouldn't be able rotate it on the x-axis because that such rotation is applied to the UpperArm bone on the z-axis instead. This is OK for basic organic boning (for human models and such) but I find it unappropriate for a mech model I'm working with since it has a shoulder part that should be ABLE to rotate on the x-axis, if I'd link it to the Clavicle. I've tried to link this shoulder part to the UpperArm bone but by doing so, I'm limitating character motion because it should NOT be ABLE to rotate on the y-axis and x-axis of the UpperArm (it will result a very odd shoulder piece because it jumps out of its joints). It does solve the 'x-axis rotation on the Clavicle problem' because I would rotate the z-axis of the UpperArm to achieve the same effect, but it leaves to many other problems.

So, the point is, how should I add a x-axis rotation on the Clavicle bone while having the z-axis rotation on the UpperArm to be disabled? EDIT: it's possible disabling the z-axis rotation of the UpperArm by locking it in Link Info.. so, my point is actually on how to enable the x-axis rotation on the Clavicle bone. :)

I've been thinking to just use regular bones instead of biped but couldn't decide to have it a go since biped seems unresistably easy to use during character animation process. Do you guys have any suggestions?

I'll have a screenshot to make things a little bit clearer..

thanks a lot for any help available!

[g]