Not real easy but not terribly difficult, once or twice a season I remove the big glass from the grain tank to give all the windows a thorough cleaning.
Yes,you can buy a long handled duster-type mop for hardwood floors that you can reach in there with.It is best to hang it somewhere under the tin work on the combine so it is always close by,and wipe the windows off at night while the dust is fresh.Taking the window out seems like it's making a simple job complex.