I'm fairly certain the "line of sight" malarkey is referring to the nacelles being in sight of each other, so they can send warp energy to each other (or whatever the theory is). It's probably in the same vein as "pairs of nacelles" idea (which unless single nacelle ships have two full engines inside the one housing, doesn't hold water anyway).
Vessels like the "Nebula Class" have the StarDrive section block a giant chunk of the visibility of the Warp Nacelle Field grilles, yet it works just fine. Ergo needing absolute direct line of sight isn't necessary.
Just look at "The Phoenix", Zefram Cochranes Warp Prototype. No need for direct line of sight.
The Warp Field grilles are facing outwards with no direct line of sight and the main vessels hull is in the way.
And the Borg doesn't need direct line of sight either and they can catch up to just about anybody.
And yeah, the warp fields that we've seen represented are bubble-like and wouldn't seem to be so dependent on the placement of the warp nacelles.
As long as the Warp bubbles can grow and converge in 3D space, that's all that matters.
Hull or Super Structure getting in the way only hurts efficiency, but doesn't seem to stop operation.
Just look at the USS Voyager / Intrepid Class.
She was the fastest vessel of her day (Yes it was short lived, Prometheus Class took over).
But The Voyager's Warp Nacelles had the Warp Field Grilles aimed outwards, even with variable geometry, the Field grilles never pointed at each other, they were both canted to face outwards.
Doesn't mean the bubbles didn't grow outwards to meet each other, they obviously did.