dave,
why do you think, that tclwebtest has a problem with richtext? Tclwebtest is a client like a google bot, and it sees just a good old-style textarea form field. from my experience, there is no need to do anything special for tclwebtest.
selenium is another story. i wonder, how well this works with other javascript based widgets. Certainly, there is a large range of things, which might go wrong in these cases.
Currently, there is little chance to have a single testing tool, library testing, tclwebtest and selenium, they all have the strengths and weaknesses... a good proxy-based testing environment would be great, since this handles xmlhttp requests as well. a few years ago, we had a student working on this, the results are not production quality....