Did you know about clipping planes? It isn't exactly what you want, but I've found it to be an invaluable tool for many a modeling situation.
Assuming you are using XSI 2.0+, click on the camera icon at the upper right of a view screen (or ALT-right-click within a view screen) and select properties to open the camera properties PPG.
There you'll see three clipping options, clip to selection, clip to scene, and retore defaults. Clip to selection works for whatever selection mode you're in, so if you're in point mode, it'll clip to whatever points you have selected. Same goes for poly's, edges, or objects.
Hope this helps.
-Brad