Sorry, I meant, “In either case, I’m curious: what would the "recoding" involve, to get around this?” (and I have edited the forum entry to correct that.)
But your explanation helps.
Then again, I would note that you may be able to deal with your need to rewrite URLs using a URL rewrite solution instead of 404 error handling. Just something to consider, and often not too difficult, but I realize some needs may be too complicated for that. I list some options at http://www.cf411.com/urlrewrite.
Finally, as for things still not working (and if you need them to), I hear you saying “there’s no more budget”. I suppose it comes down to a simple economic decision. How much would they pay you to do the “massive rewrite” you refer to? I suspect more than a few hundred dollars, right? 
But we may solve the problem in less time than that cost—and if we did not, then I said you would not have to pay. That’s my satisfaction guarantee. (Obviously, if we didn’t solve it within a couple hours, and you were not prepared to pay more than that, then we would stop at that point, and you could proceed with the rewrite.)
Just trying to give you an option short of that rewrite. Hope it may be helpful.
/charlie