Poor form to use domains for delegation?

[MattK]

Doesn't look like there is anything in the spec's that dictate a
delegated container should be a zone not a domain - but is this
acceptable...

Some background:
Split horizon DNS - inside / outside
Parent zone example.com authoritative on both DNS.
Sub zone sub.example.com delegated from inside to outside
Outside DNS hosts domain sub.example.com

When I do a recursive query to inside for NS sub.example.com I get
NXDOMAIN, although when I query for RR's in sub.example.com I get
results.I am not replicating sub.example.com - so is there any
requirement for SOA and NS records for this domain on the outside DNS?