Secondary DNS for Paraguay's TLD?

  Most "real" providers have diverse nameservers. For them,

Correct. But diverse announcements are neither a necessary nor
sufficient condition for diverse namespace. And given you can't
(in Europe anyway) get diverse announcements from day one, and
you said...

> IMHO, the internic should not allow any domains to have pri+sec nameserver
> in the same /24

...your proposal would exclude people with valid subnetting plans
but no diverse announcements who wish to register new Internic names.
Given Internic's current situation with lame delegation, I think
this is the least of their problems.

If you wanted to fix this correctly, you could find the penultimate
hop to each nameserver, and check they were different. This would
seem to be closer to what you are intending to achieve.