I have a feeling that the answer is going to be 'You can't do that without some very complex coding' but I thought I'd go for it anyway.
I recently asked Creating Polyline Buffer in OpenLayers 2?, and it worked! Well, almost. In our mapping application Dorset Explorer we have a line measurement tool which works using Geodesic measurements. It had to be like this as non-geodesic resulted in massive errors (I measured it against the 100m running track at my old school and it returned 125m, which I know was wrong!). But simply setting geodesic to True in my OpenLayers measure code fixed this.
I now want to create polyline buffers of a user specified size in my OL Map. I have managed to get it to make a buffer using the answer to my first question, but the size is wrong. In the attached image I requested a 100m buffer around a polyline going from left to right (which is not visible). I then measured the size of the returned buffer (in red) and came up with the result 125 metres.
Not good. I wonder if anyone else out there has attempted to make Geodesic polyline buffers. Even a calculation that would allow the user to put in a 'Geodesic' length and return a 'non-geodesic' length for the buffering tool to use, but I can imagine that this is near impossible. It's driving me slightly nuts, it's so close to working but this rather major issue has stopped it. If you want any more info to help answer or point me in the right direction please let me know.