you will have the same issues with a chain tension with a coaster that you would fixed. You need to find a magic gear and tennsioners aren't an option.

You should just build a fixed gear or a ss with a front or two brakes respectively. Probably an ss since you can just get a simple chain tensioner to deal with the tension. Looking cool is only worth sacrificing safety if you are a trend chasing ****up. Looking cool is clearly the only reason you are doing it since a coaster rules out all the other ridiculous arguments against having a front brake.