Although it’s not the most semantic solution, couldn’t you just create to divs, both containing the “6” where on has color: red and the other color: white & the filter and then just stack them with a bit of displacement?
Any comments/improvements on this? The idea is that it goes into a separate stylesheet with a conditional statement. All other browsers will be using the modern implementation (which isn’t shown in the pen for demonstrative purpose).
Unfortunately we run into the same trouble when trying to apply a blur – the special IE filters do not seem to work at all anymore. The only way to reliably implement a blur would be to wrap the text inside an svg and apply a Gaussian deviation, which would really lead to far. It does look a lot better (than nothing at all) with a pseudo element already though. And using hsla, supported in IE9, can help a bit as well.