[XeTeX] Monospace font, not being set in monospace

[mskala at ansuz.sooke.bc.ca]

I'd like to use the commercial font "Pro Typewriter Underwood," from
http://www.vintagetype.com/, with XeLaTeX, to produce a close
approximation of old-style typewriter output.  But I can't get it to
correctly recognize that the font is a monospace font for the purposes of
inter-sentence spacing.  "Word" spaces come out to the same width as
non-space characters, as they should.  But each "sentence" space comes out
to about 1.35 times the width of a "word" space or non-space character,
when it should be exactly 2.  As a result, the text no longer fits the
grid, and the purpose of using a monospace font is defeated.

Although there's a specific commercial font I want to use, I think the
problem is actually internal to the fontspec package, because it happens
with every monospace font I've tried including whatever the default is.
This file demonstrates it:

   \documentclass{article}
   \usepackage{fontspec}
   \begin{document}
   \noindent
   \texttt{x. xx\\
   xxxxxx}
   \end{document}

The two lines should be the same length, but they aren't.  The
space after the period, which should be as wide as two xes, is
actually about as wide as one and a third of them.  Comment
out \usepackage{fontspec} and the lines become the same length.

I'm using XeTeX 3.1415926-2.2-0.9997.4 and 2010/09/19 v2.1, both from
TeXLive 2010.

I took a look in fontspec.sty hoping that I might find a place where it
says how much space to insert between sentences, which I might adjust, but
it seems not to be that simple.  I tried playing with the \sfcode settings
as might be appropriate in regular TeX, but I couldn't get them to have
any effect; I suspect that whatever fontspec does is overriding that.

It occurs to me that since I only want to use one font in the entire
document (basic typewriters don't have selectable fonts...), maybe I don't
need fontspec, but could just manually simulate its effect by defining
some macros of my own, Plain-TeX style, to serve the function of loading
the one font.  Is that route likely to be viable?
-- 
Matthew Skala
mskala at ansuz.sooke.bc.ca                 People before principles.
http://ansuz.sooke.bc.ca/