It’s not feasible. There are plenty of free secondary dns services out there.
most I’ve seen, are pretty restricted actually - mutual DNS however would be a win/win for all parties involved, and would not need to be restricted in the same sense. Also, such a “give & take” arrangement would not need to be restricted to just bind/named (DNS) - it could involve other services that would benefit from redundancy, such as postfix or httpd clustering.
Fully agree with you Wocul on share capabilities between all Virtualmin users, it would be really great 
as your RPC solution is nice but it stills gives remote site full access to your Bind configuration which is not very nice !
What you should test before doing this in production use is whether via RPC that new user can do more than those things you defined with local access rights.
It’s sure possible that I’m mistaken, but I seem to recall that, when I asked those same questions a while ago, the Virtualmin team told me that via RPC, the local module access restrictions don’t apply.
It’d be nice to see though if I’m wrong there and the restriction to the BIND module DOES apply also for RPC calls. I’d do the same for my DNS cluster slaves then!
vincen please refrain from spreading false info about RPC – its rubbish and your only causing panic for no reason. RPC IS NOT A SECURITY ISSUE