Ok, had a quick dabble tonight, was less work than I imagined, mainly because the options for UI for device plugins is fairly limited
Attached is a patch file Kovid. I leave it to you to change the descriptions etc if you have a better way of expressing it. The behaviour is that if a valid custom column name is specified in the Kindle device configuration, *and* it has a non-zero value in it, then that page count will go to the Kindle. Otherwise it falls back to the existing behaviour of computing a count.