I'm actually surprised this isn't already a built-in variable. #CURRENTCONFIGMONITOR# returning 1, 2, 3, 4, etc. depending on the monitor the skin is currently on.
with AutoSelectScreen=1 in Rainmeter.ini. SkinWidth= does not use the AutoSelectScreen value when Rainmeter is opened. Only if the skin is later refreshed after open. Continues to use Work Area Width of the primary monitor. Incorrect Work Area persists. Would be solvable with [#WORKAREAWIDTH@[#CURRENTCONFIGMONITOR]].
I am not opposed to this in general, but honestly, I am not too sure how beneficial it will be to most skins. Plus, it might be difficult to determine which screen to return if the skin spans across multiple monitors (either very large skins, or when KeepOnScreen=0).
The real is issue is the time it to implement this, which seems to be ever decreasing lately (at least for me).
There are some important projects I would like to really get done before too long, so unfortunately, this might be put on the back burner for now.