Only, that ellipsoid shape in ST2:TWoK was the Klingon Empire.
Plotwise, that's not very satisfactory. After all, we know the Romulan Neutral Zone has not gone away in the meantime. The heroes should not be talking about "the" NZ if two of them brush against Gamma Hydra - the potential for confusion would be too great.
OTOH, we do know that e.g. Khitomer changes ownership, from neutral to Klingon, at some point: Klingon borders certainly aren't explicitly immutable in any given time period. So a KNZ reaching up to where there was nothing but the RNZ before is a possibility at some timepoint. But only at a timepoint after the incident where there's only a single NZ near Gamma Hydra...
Or should we postulate that the RNZ indeed was briefly disbanded, or relocated, in the late 23rd century (a time of detente where Romulan Ambassadors are allowed into the Federation's most secret strategic meetings)? This would IMHO take much more doing than saying that Klingons can be found inside the RNZ - either only in cruel simulations, or then also in the reality of the universe, wreaking havoc on Romulan reputation by preying on third parties on their territory.
Timo Saloniemi