Custom Transport Agent

Does anyone know of any organisations that can write custom Transport Agents for Exchange at all? Our customer wants to configure Exchange so that a certain group of users can only receive email between certain hours, and email sent after those hours is held in Exchange and not sent to the mailbox until the next day. They want to do this in an automated way, so configuring for moderation is not really an option.

I've done the obvious Google searches, but can't seem to find anyone advertising this service.

Thanks