> I do not want to have the code in a package. This would
> result in a patch. A hook wouldn't blow the kernel up, the begin
> document hook is only a few lines of code.
even if it was 1 line of code (which actually it is as there is a begin
page hook, but it disables itself after the first time, so it can be
advertised as \AtBeginDvi) you would want it as a package.
making changes to the kernel to add new features causes so many
problems for portability, we still now have to deal with the affects of
allowing \MakeUppercase to work on section titles: a change made 3 (4?)
years ago. A package, especially one that contains a single line of code
is far easier to manage in terms of document portability.
Of course technically there is no reason for this: anyone who can
download a package can download latex/base and make a new format
but real life unfortunately doesn't follow logical rules.