I think what Giulio is referring to is that the part is made normal and then everything is mirrored. What we need is a way to make the part using a mirrored universe (I just heard 10 programmers drop to the ground!).
At the very least, give us control of the Mirror-Merge so we can use it on the fly by deleting it, making changes, and adding it back.
Giulio, in the thread I linked above, PTC support stated how you can preserve the drawing for mirror parts. If that doesn't work, let us know and we can see if PTC can help you with that also.