> In any event, do I understand that the current XF86-centric termcap entry
> does _not_ work with a stock R6 or R6.3 xterm?

Yep.

> Has anything significant changed between the R5 and R6 XC xterms?

No, not as far as the terminal emulation capabilities are concerned.
If I understand the previous message in this thread correctly, the XF86
version has implemented another bunch of VTnnn escape sequences.

-pk