I understand your problem. But as far as I do know, it cannot be solved with mkgmap styles. Also not with “simple” database operations.
The complicated point is that a “way” need not fully be inside an “area”, and in such a case you’d have to split it into way(s) outside the area and a way inside. But even for the simplified case that at least one point of the “way” should be inside the “area” to be considered inside, there is no solution with mkgmap, and with Microsoft Access you’d have to do lots of VBA programming (and with the thousands of ways and areas in the Netherlands, the iterations would be inacceptably slow).
