can't you parametrize the hole diameter and the fastener diameter independently or concurrently? that's one way. I don't know how it works in Mechanica, but some FE fastener models automatically specify fastener diameter when you create it--whatever hole the fastener is assigned to, the FE software automatically assumes the fastener and hole are
'neat fit', that is, the same diameter. To specify a fastener hole clearance or interference, you might specify that someplace else like the Load window/interface.

That's basically what I ended up doing. I created a Pro/E Parameter in the assembly and then related the diameter dimensions to the parameter and then in mechanica related the fastener diameter to the parameter. Then to top it all off I made the parameter a design parameter for Design Studies and Optimization.

UPDATE:

Turns out there's some kind of bug that stops optimization studies in their tracks when doing this. If you use fastener shear stress as a limit it causes a problem. The measure on the fastener shear stress disappears between steps and then the study fails with an internal engine error. If you bring the definition up the limits on fastener shear stress are missing.

Global studies work fine.

Short version: You can't optimize based on fastener shear stress.