Gmail just ate my reply.  You all win.  I will be brief :)

INF file:  CCS supplied one with their USB.  Seems Microchip should, too.
Someone who knows the structure can probably make one.  It basically says to
use whatever.sys after recognizing a VID:PID

Will it play?  CDC is actually very involved, so a change in a descriptor
could ruin it.  But so far, success rate is surprisingly good.

USB in your firmware:  I "spliced" the CDC character and string functions
into my own serial handling functions.  Then I was able to run all my old
serial apps over the USB.

Barry
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