I think Bumper is right James unless you want to go down the WFP route for the outsides. Your problem with a buffer is a simple one really, "square holes and round pegs" It's not going to work mate. Your buffer will be round and your window isn't
So even with a buffer of some sort you're still looking at scrimming the edges mate, although now that I mention it, you could try a very soft 3" buffer head. That would push into the corners and probably do ok, but my concern is the possibility of damaging the frame surround.
I have an air powered mini polisher that I brought for paint work correction, I've only used it once years ago as my compressor was not up to the task of running it
That could work
But then you're looking at a compressor or power fed polisher, plus the mops (buffing pads) and to be honest I think you'd do it quicker with a scrim
What really puts you off the WFP for the outsides? The fact that you leave them wet?