Currently, my @mainpage looks like:

...
Document generated on: ${DOXYGEN_GENERATION_DATE},
branch: ${DOXYGEN_GENERATION_BRANCH},
revision: ${DOXYGEN_GENERATION_REVISION}
...

Now, of course, doxygen does not know anything about DOXYGEN_GENERATION_DATE,
so my options are:
- use sed to substitute values in the source, before using doxygen (which is what is done currently),
- use sed in the generated output (ok for html, harder to do for pdf etc) to change values post generation.

The feature request is to have a new command, say @environment,
so the documentation can be written instead as:

...
Document generated on: @environment DOXYGEN_GENERATION_DATE,
branch: @environment DOXYGEN_GENERATION_BRANCH,
revision: @environment DOXYGEN_GENERATION_REVISION
...

Before invoking doxygen, the user build script is to export values, for example:
export MY_CUSTOM_THING=<my custom value here>

Doxygen, upon finding the tag @environment MY_CUSTOM_THING, is to execute a getenv("MY_CUSTOM_THING") and expand the result in the generated documentation.

Thanks for considering this request.