You can already now associate a settings file with a solution (right click solution).. so I would suggest setting up something that could use that.
Yep I saw that feature, nice, but that would be the file to use for the options to check against the code. What I'm saying is the way of programming the JS/CSS check at build without the need of installing the tool since for example TFS Build won't have
it. And another situation is for example where only a few developers can configure the options to validate of JS and CSS and every other developer doesn't need to have installed the tool but is needed to be able to check de validations.
Possible solution is like StyleCop team done it: checkin the dll used to validate, edit project files of the solution to include an import tag with the dll used to validate and include a targets file where build indications are specified.