I have been outside pokin and proddin on my 'Stream in search of an idea.
I would not want the front window guard open while driving either and I absolutely would not want to remove it.
On mine the Kitchen window is by far the most sturdy of the bunch.
My front window and the area under it flexes very easily despite there being some additional bracing there for the standard table mount.
The side window you ask about is even worse on mine as there is nothing at all supporting the fiberglass above the level of the bench and the wall around the window is very bendy.
I could picture installing some kind of horizontal sill plate to give the AC more solid perch and prevennt the wall under it from flexing but this would stick into the bed area a little but could also make a nice nightstand of sorts.
The Kitchen window however is solid like a rock as the cabinet and Bathroom wall and the Divider on the other side help to frame it in and provide support.
On mine I can not even cause a deflection in the wall from the outside under this window.
There is also power there and it is the same size as the other windows
I imagine it would block your view a little and will stick inside the trailer where work is actually done but it feels MUCH more solid as a mounting surface if you will be leaving it in place.
I would also think it would be simple to make some kind of detachable leg to support the front of the AC inside over the bench if you really want it there. It could run from the front edge of the unit down to the bench for travel both supporting it and retaining it in the window as an added safety.
If the unit only sticks outside 6" as you say I wouldn't be too worried about it hitting things but it is another thing always in the back of your mind when towing.....and my mind is already crowded enough! I have certainly seen them mounted this way though and never heard of it being a problem. In fact I have also seen older units that sat almost entirely outside mounted this way without problems.
On another note,I never fail to be amazed every time I walk back inside the Fiberstream
just how spacious it feels inside.
is very cozy and new but the Fiberstream
really does feel twice as big inside.