This is a follow up on this earlier post.
Today the release version of Windows 10 IoT core became available. So I immediately downloaded it to check if anything changed about the usb serial support..
And I'm sad to report that my usb devices (pl2303 and ft232) still don't work.. BUT, I did found a forum post that claimed that the silicon labs cp210x series is supported.
So I ordered myself 3 of those, and will report back when they are delivered.
found a reference to confirmation here