I've always been intriqued by Jefferies (Not the "JeffRIES" I have in my title LOL) smaller Enterprise concept and have always wanted to reconcile this with the existing Star Trek universe. So, while working, it popped into my head to use the NX class saucer as a launch point. Something that came after the NX Class refit. I modified the saucer, enlarged and modified the nacelles, and drew a quick dirty sketch of the concept secondary hull. This is all dirty so far. I may lengthen the secondary hull a little, and proportion up the nacelles to it.
Actually, I had been thinking along similar lines for several years now. We never got to see ENT reach the Earth-Romulan Conflict; "These Are the Voyages…" overshot the war. My thoughts on this early Jefferies design for the TOS Enterprise: In TAS "Time Trap", it was mentioned that the Starship Bonaventure was "the first ship with warp drive installed". Curiously, the characters of ENT never referred to their "Warp 5 Engine" as anything but an "engine"; never "warp drive". In "These Are the Voyages…", Tripp and Archer toast to "the next generation" of ships that will be able to achieve (hopefully) Warp Factor 7. Warp 7 seemed to be almost a Holy Grail in ENT, like joining a club of spacefaring civilizations whose ships could cross this velocity threshold. I would suggest that, just as Earth's "Warp 5 Engine" was a technological milestone for entering the interstellar realm, so we can assume that Warp 7 engines represent a new technological plateau for Earth (and the Federation) to climb to. Thus, Warp 7 engines would be considered to be the first generation of true "warp drive". So maybe the Bonaventure was a crude first attempt to reach for the brass ring of Warp 7, with an early attempt at true "warp drive". I'd like to think this early Jefferies drawing could be a useful guide as to what a rough, early prototype of first-generation warp drive starship (which would ultimately evolve into the Constitution-class of Federation starships) could look like. Keep going with your concept. It's really neat!