Hey!
That's a really good idea, actually! I'm still in the process of having a better outline/dropshadow shader created by a friend of mine, so I'll forward this to em! Right now, the dropshadows are just the main text pass, but with a different colour applied...
The way I'd get this working is to, as you said, add a blur slider, directly in the shader. STM uses fragment shaders, and there's plenty of ways to do gaussian blur in a fragment shader, so... I guess it's a matter of trying to make the two play nice. I've already messaged my friend, so we'll take a look into this on the weekend!
-Kai