As far as I can see there are two possibble approaches that we can take
(there may be more):
1) To have a "shrink to fit" attribute, which would cover both the need to
reduce the width of the page image of a US Letter formatted document to fit
on the more narrow A4 media, or to reduce the length of an A4 page image to
fit the shorter US Letter media.
2) To instruct people to use the "best fit" option for the media attribute
and hope that the printer is clever enough to figure out that it has to
reduce the page image to fit.
Whichever way we go, I think it is important that we have an answer on this
for the upcoming IETF meeting, as well as put it in as an example in the MOD
document.
My 2 cents..
Carl-Uno