We have done quite a few matte black cars with avery 900, these don't tend to need special care, mine has been done 2 years ago and I drive it trough the carwash at least once a month, still looks great.
We brand new Mercedes E class convertible in Hexis matte white, and our demo Nissan 200SX is also a print with matte laminate, that has a lot of white.
Both cars are a nightmare to keep clean.
After trying every product available to be able to get the white white again (after a few weeks outside the white is brown because of the dirt) I've found one that does wonders: Procar Diadeem universal cleaner.
Matte white goes back to white, with no shining.
After talking to a detailer, we ordered a Swisswax "Opaque" kit
this is a wax specially developped for matte paint/vinyl and a quick detailer.
the starter kit (for 4 wax jobs) costs 150 euro's !
It claims black goes "darker" but trying to do only my hood didn't give any difference.
what it does goed is repell dirt, waterdrops washes runs right of (like on a regulare waxed car) so no dirt gets a chance to stick.
maybe doing a swisswax waxjob right after the rap might help keeping it nice, but that would mean rising the customer pricing by a lot
(the swisswax wax should be waxed in with your fingers, takes quite a while
)