There is a configuration option for dotLess called "InlineCssFiles" that I believe does what you need. You can use your own configuration by overriding the static property "EngineBuilder" (its a Func) on less preprocessor. So in App_Start you could do something like this:
LessPreprocessor.EngineBuilder = () => new EngineFactory
{
Configuration = new DotlessConfiguration
{
InlineCssFiles = true,
}
}.GetEngine();