As for your latter suggestion - I think its unrealistic to build a general web-based solution. As you are describing it, you would essentially be hosting R, some visualization tool, and a scripting tool. That's absurd enough and doesn't even address the bandwidth issue - these datasets are usually huge.

You'd be better off writing enterprise software in this field. Especially considering anyone who would pay for data munging and visualization tools/services probably doesn't want their data out on the internet.