Allow an administrator to set an arbitrary http return code for maintenance mode

It would be very beneficial for me to be able to set an arbitrary HTTP code to return when a site is in maintenance mode, which would not trigger monitoring that a site is down.. and wake me up at 3am.

There are SEO and other considerations that are at play as well here, so in many cases the default 503 might be the correct response, however as an administrator I would appreciate the ability to change this.