This is actually something that we've done in the past, but I don't think we've ever dedicated an entire server for it. We have had several "Custom" servers in the past that have had foot races on it, but over time, we just found them to not be as popular as the regular hog racing servers. It can be a nice change once in a while, but people have always tended to go back to what they normally always played
The other thing about foot races is that there tends to be very little teamwork. Even though you can be contributing to your team getting laps, there's just not as many chances to pick up teammates and work together like you can when you're in a hog race. Having a team aspect has always been very important for BFM, which is probably one of the reasons you don't see too many foot race maps around anymore (on our servers at least).
We do have a map on our ROCKS server that has a foot race on the map Chill Out. I believe that's the only rotation we have at the moment for footrace though.