As far as I know this is not possible. I did some tests and unless there is a way of manipulating cutting planes that I am unaware of it wont get the outcome you want. What I normally do when I have parts that need to be shortened is make a configuration of the shorter hose in Solidworks and then import this new part into my Composer file.
Thanks. That's what I figured. I ended up cutting the assembly in SW then moving stuff in Composer. I'm learning as I go with Composer so when I find something that seems simple I just figure it's buried in there somewhere. It'd be nice if I knew it wasn't, that way I could build my bag of work-around tricks faster and move on.