Would this cause problems ?

[Jonathan de Boyne Pollard]

SH> What I thought could work would be :

This is "split horizon" DNS service with multiple databases, 
one of the ways of setting up "split horizon" DNS service.

<URL:http://homepages.tesco.net./~J.deBoynePollard/FGA/dns-split-horizon.html#MultipleDatabases>

SH> In an ideal setup, each admin defines the domain names used
SH> internally and gives these 'internal roots' as the master 
SH> servers, [...]

There is no need for this.  Since (as you say) the delegations in 
the public DNS database will point to those same content DNS servers, 
there is no need for overrides.

SH> I suppose the big issue is the "correctly setting up the 
SH> routing, and views" that I so casually throw in !

The important point to remember is that client differentiation
at content DNS servers, such as "views", operates based upon the 
IP address of the resolving proxy DNS server, not upon the IP 
address of the original DNS client.  In other words, your "views"
must be based upon the IP addresses of the (back ends of the)
proxy DNS servers that your administrators are running, not upon 
the IP addresses of the machines that they themselves are providing
DNS service to.