Can a theoretical proof-of-concept be published? I am working independently on an idea, and to test its conceptual feasibility, I have done a bit of math, and it seems to check out. The idea is in the area of web engineering, or web science.

Now, in order to simplify the math, I've had to make several assumptions, which may or may not be valid in the real world. Is this a problem? I'm trying to make the calculation as realistic as possible, but I've noticed that it can quickly get out of hand in terms of complexity of the problem.

However, I believe that the concept has value in that it is a novel way of approaching an existing problem, and it could have the legs required to flourish in a theoretical sense.

EDIT: Just to be clear, the simplification I'm referring to replaces non-linear functions with linear functions, so that the math is simpler.
From your description ("I have done a bit of math, and it seems to check out"), it sounds like you have ended up in an entirely ordinary and reasonable piece of theoretical/analytical research.  Indeed, such research can be almost precisely characterized (informally) as doing a bit of math and seeing if it checks out.

The question of publishability then boils down to whether your simplifying assumptions are considered reasonable (which you can argue and test by comparison with prior work, both theoretical and experimental) and whether the conclusions reached by checking it out are interesting and significant.

Bottom line: go forth and publish (in an appropriately scoped venue)!