Pierre, Thanks for your note.
We are having Seattle weather here!

I had run into the \hidewidth problem too. When I changed \newskip to
producing a \chardef instead of an integer, for ``efficiency,'' I
also clobbered \hidewidth unwittingly. Here is the way it's now
defined in plain.tex:
\newskip\next \setskip\next=-10000pt plus 1fil
\edef\hidewidth{\hskip\skip\the\next\space}

The       Globals      in     the     outer     problem
was also fixed in the latest WEBHDR. All I had to do was remove
\hbox in the definition of \X (or something like that). As far as I can
see, there never was any point to having \hbox in the first place,
but it must have been a vestige of something.

I hadn't noticed the problem with \b, because I guess it was on the
bottom line of my test page! Your solution can be slightly simplified:
	\def\barbelow{\hbox{\vbox to .2ex{\hbox{\char'22}\vss}}}
\vss and \hss generally add whatever glue makes the other dimensions
come out OK.

To take care of slants, I haven't had time to test this, but I believe
something like the following will almost work:
	\edef\next{\texinfo\the\font 1}
	\setcount0=\the\next % now \count0 = slant per point, in sp
	\divcount0 by 1024 % scale it down, to avoid overflow
	\setdimen0=.2ex
	\multdimen0 by \count0 % now \dimen0 has 64x the adjustment we want
	\moveleft .015625dm0\barbelow
all of this to replace "\barbelow" after \hidewidth.

David says a slightly newer WEB manual is on its way to you. We haven't
released the latest version of the manual, because we're still
debugging it as TeX changes little by little. I'm almost done with
Chapter 20 of the TeX manual, which means that the process of
small refinements is nearly complete at last. Oh how long it has taken.

As far as I know, nobody else has laid claim to DVI commands 250 and 251.
A note in TUGboat by your group would be the best way to establish
your claim; you should have some distinguishing ID number for your
brand of DVI files.

Best wishes, Don