But flattening and allowing a bit of trailing edge margin (> 1") seems to be important.
I think that's right. I installed the P800 drivers to see if there was any changes in platen gap when using roll paper, and there didn't seem to be. So there's something about the P800 which means you don't get this problem with that particular printer.
If the printer really is very similar to the 3880, then rather than the design of the paper path, I suspect that it's the leading and trailing edge preventing problems. Based on smaller printers, I think there's roughly an extra 2" used at the front to ensure that the leading edge is feed into the exit rollers and hence the paper is flat. Since it's on a roll there is a built-in trailing edge at the print the printing stops. I've had some issues with the leading edge, but as I was printing on a longer than necessary cut sheet, I didn't have any problems with the trailing edge.
Could a P800 owner please confirm how long a lead strip the P800 uses when printing on a roll? Is if around 2"?