I think I may have a rough Idea how to do it now.
I was thinking a func_rot_button which will be rendered invisiable.
This will cover the steam jet and have have the health set so it will be shootable.
Also there will be 2 triggers either side of it so when they are passed through the func_rot_button will move out the way as well as disabling its 'target' feild with a change target. and when the trigger is activated again it will come back down.
There is obvoiusly a dead zone with this, but it could be cleared up with more func_rot_buttons and more triggers.
But I think this soloution is more than complex enough anyway and it should still give the same rough illusion.
Plus who would want to stand in the steam jet and make it explode anyway (if their not suicidal).
Thanks for all your help guys