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.
Production: NX10.0.3, VERICUT 8.0, FBM, MRL 3.1.4 | TcUA 10.1 MP7 Patch 0 (10.1.7.0) | TcVis 10.1
Development: VB.NET, Tcl/Tk Testing: NX11.0 EAP, NX12.0 EAP
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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.