Dear Ross!
Last night I found much more elegant (obvious?) Xy-pic solution to
that problem:
\entrymodifiers={+!!<0pt,\the\fontdimen22\textfont2>}
See some examples at http://www.mccme.ru/~vadim/examples/test.pdf
or just try \xymatrix{\widetilde W \ar[r] & W} with/without it.
I wonder, isn't it the desired default behavior for xy-matrices?
Yours,
Vadim