As far as I know, the only thing that will data reference directly with C3D, is C3D (but only the version you are working with). I've had to convert Carlson, Eagle Point, Microstation, LDD, and even C3D (different versions) for our engineers, in order to make the data work for them (when they get outside survey). A good thing about C3D is that working with surfaces is pretty easy, so recreating a surface in C3D (from tin lines or xml) is really fast.
You are right there. Even going between EaglePoint AutoCAD and MicroStation, we always had to TIN the TIN until XML. By then we had been so used to TIN'ing the TIN. we just kept on doing it.
I'd ask Woneal, how much of the post field survey workflow does the survey-side need to do before the handoff to engineering? A roadway centerline is usually independent of a sectionline, so leave the survey work in an AutoCAD base drawing that the engineer's XREF into their C3D project drawing and "no touchie" from there forward.
I watched a large interstate project get corrupted by another office's engineering group. I'm an engineer myself, but our office had been working from a copy of the original 3D survey drawing in MS EaglePoint rather than their corrupted MS GeoPak 2D base drawing. Somewhere along the line, existing underground utilities dropped out of their drawing. WTF?, I get called to the carpet for connecting inlets to non-existing storm sewers that their folks had deleted. Nope, I'm looking at the original point file in the drawing showing EX INLET, what your drawing shows nothing? Hmmm, which office gets to eat the rework?
Woneal, if I understand your situation correctly (and maybe I don't ?) the survey office has an existing workflow, that works but is just not the latest and greatest AutoDesk C3D? I assume they make money at what they do? How hard would it be to let them keep doing what they're doing and let them hand you the following:
(1) AutoCAD base drawing (no AECC elements, but a generated TIN mesh that C3D can TIN from)
(2) a single ASCII point file, exported from their final drawing all blessed from the PLS, so your office can import the points as AECC points
Your office could then XREF the survey drawing into engineering's C3D base drawing. Engineering generate a C3D TIN from the surveyor's TIN mesh in your drawing and import the single ASCII point file as AECC. Done with little to no dollars outlaid and a LOT of headaches spared.