Enhancement that allows you to execute arbitrary code *before the job is put on
an executor* and periodically by specified interval. If exit code is non-zero
then don't put on an executor. If zero, then execute.

You don't rely on simple locks but you can check for example there is enough
time before database maintenance window or something like that.

I think most flexible would be to allow for execution of arbitrary available
task in hudson. Though most useful will be a groovy script executed on the
master system. That will allow for one to check hudson state in any possibly
needed way, as well execute external command if needed.