"cut to plane" is the arch command,
all in all it is 5ish clicks to make the planar cut (of one extrusion)...
it all boils down to that cad is not sculpting clay, it is more like graphical programming of geometrical definitions, so to modify a dead geometry one has to go to nuts and bolts (vertices and lines), or simply redo the whole thing.
if you are somewhat familiar with fc, remodeling your reference model is 10-20 min work,
and then you can alter it with better tools than using vertices and lines and rebuilds...
maybe do one or two of the Tutorials for modelling on the wiki if you do not feel like you can remodel the reference model in less than half an hour - spending some time on those tutorials is probably better than to try to find answers on what you think should be possible (since it might work completely different to what one thinks it should
).btw, the cut plane command is nothing else than a "built-in macro" for making a bool-cut, so you can do the same with making a box yourself and position it and doing the cut with the home-made box.