rl_returnz is actually pretty stable (mydomain.com is running it since quite a while ago with no trouble) and I'd recommend enabling it by default if you make it configurable. It would give first-time OpenACS users a nice "wow, this thing is fast!" feeling :)
I don't think this solves David's problem though, because the browsers crash/hang when trying to rendering the 5Mb HTML, not when downloading it.
