Mark, What I am calling platen release is done with the front panel upper right corner button (shown with an arrow in the crappy photo below) that you'd press to entrain roll media. The Epson manual alternately uses the term Paper Release for this button.
When I first got our 4900 in April 2012, I called Epson with the complaint that I was having trouble feeding heavy weight fine art media from the Rear Path. My finding was that after placing the sheet in the rear slot until it bottomed-out, pressing the Down Arrow did not entrain the sheet unless I manually applied a gentle downward force on the sheet at the moment that the downward arrow was being depressed. Epson conceded that this was a known problem with the 4900's rear path, and I was instructed to load the heavy paper with this other method of releasing the platen. I think is was Hahnemühle William Turner and Canson BFK Rives that was giving trouble, but so also was Epson Exhibition Fiber - so Epson couldn't blame the 3rd party paper makers. Epson told me in that call that they were unaware of any plans to *fix* the 4900's rear path problem. Since that time, I've just been using the down arrow and nurturing the loading of each sheet, with only occasional problems.
Fortunately, the cassette paper path works for most of the PK papers I print on - even heavy ones like the the 335 Innova Warm Cotton Gloss - so I don't need the rear path for high volume printing. Do you, Mark, ever need the coax a sheet into the rear path with manual downward force on the sheet? Or does the down arrow entrain basically anything you've placed in the rear slot? I wonder if there is any adjustment that could allow the transport to grab sheets better on my machine...
As for the original problem of unstable sheets during the printing, I'm stumped, and have no idea why this alternate means of sheet loading seems to obviate the skew during printing.
Thanks for your interest, Mark.