OK, I've put together two new contexts that might be useful to you. One is a PolynomialFactors context in which you can enter products of powers of polynomials (and nothing else), and the other is a RationalFunction context, in which you can only enter rational functions (or optionally products and powers of them). These are in the files contextPolynomialFactors.pl and contextRationalFunciton.pl in the macros directory (you will need to use CVS to check out the new files).
These are not heavily tested, but seem to work for me. They are based on the LimitedPolynomial context, which has been around for a while, and so has been tested more thoroughly. You will need to check out the latest version of that, too.
Read the comments at the top of the file for details. You may want to set the strictQuotients flag to disallow products of rational functions if you want.
Anyway, hope that helps. It's something I've been meaning to do for a while.