To the best of my knowledge, fields are not available for use within any C3D labels. If you're looking to use equations, have you tried using expressions?
I use expressions for a bunch of things, but I don't think it would work for the other situations. I extensively use fields for things like site plans.
For example, I'll have a block reference comprised of MTEXT that has the following:
-name
-area of the building (closed polyline)
-conversion of m2 to sqft (rounded to nearest 25 or 50 sqft)
Then for the data matrix, I'll use MTEXT for the following:
-lot area
-equation expression that auto adds all the building areas
-equation expression that auto adds all the asphalt areas
-subtraction of the building and asphalt to get my landscape areas
-%'s throughout the chart referenced from those values
-an auto tally of parking stall numbers with conversion to % based bicycle parking
I'll have MLEADERS that do similar things (ie. max ponding limits on a 100 year storm). If I could do it through expressions, I believe I'd have to make a new one and name it each time. It seems like the note idea is awesome for existing features; not so much for proposed "math based" ones. Though, that's likely me being unfamiliar with how to make expressions target individual objects.
Ie. Surface ponding example:
Volume = (area of ponding * (OFR spill height - Lid elev))/3
I can select the area, spill height, and lid elev using fields. I don't know how to do that with expressions.
Thanks for the reply!