The reason we don't hear about oodles of .400 and .465 H&H's around is because there probably aren't many.

They are most likely quietly making a few rifles for their clients who request them. For some very respectable prices too.

They certainly do list the calibres on their websites for a custom made bolt action:
http://www.hollandandholland.com/~newyork/newguns/bolt-action-mag-rifle.htm

The fact that 'Remchester' is not chambering the cartridges and will never do so, probably doesn't phase H&H at all. In fact I am sure they like the fact.

As long as brass is available and dies and maybe loaded ammo I am sure they will continue.

Just because you don't see as many Morgan cars around as Toyotas doesn't mean they aren't a successful product for the manufacturer.

Personally I really dislike any cartridge with the name "Remington" in it. I think a .400 H&H Mag in a custom rifle (non H&H rifle and non H&H price tag) would be cool. Just because not one else has one.