Is there an easy way to do this?
I have 2 smaller (400mm) 5 axis machines and I'm always fighting cavity mill to stay inside my Y travel limits when tilted over at a-90.
Which NX release?
Starting with NX 8 or 8.5 many operations offer to force engage to be kept inside the trimming boundary.
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Machine limits won't be recognized until you simulate.
If you want Cavity milling to avoid the limits when the path is generated, I agree a trim boundary is good. You could define a rectangle at the limits and set the custom stock to a little more than your longest engage length.
If you to use it for several operation, put it in a mill area geometry group, under the workpiece.