If you have a small group that wants to build a following then I suggest working on developing your own system to account for down wind only. I would use Portsmouth as a starting point and work into rating that all parties agree is fair.
First understand that none of the current rating systems can or will ever be "fair" as you are expecting the impossible. Second realize that no one has a real handle about what is the answer why one boat is fast in one direction and another one is not, yet change the sea state a little and the reverse can happen. As an example, look at the last America's Cup race. Do you really think they thought the cat would be that much slower than the tri? As another example, which rating system would you use if they gave your little group those two boats so you could race them now? Also, lets picture that you were given the ratings before the race took place, what would the rating have been? AND, would they have changed them now that the race has been run? Was the difference in speed, the boats or the sailors?
So back to the basic question, how to have a rating system that is fair for all the boats using systems for up wind down wind on a one direction distance race? Have all parties agree that the system is flawed, but by agreeing to work together you can develop something that your group can all agree to have fun sailing with..
I will warn you though, in the past I have tried something like this, where a group of us sat down and each got to pick what rating they felt they could be competitive at...what I learned form that, was there are really people out there that do not want fair. They want an advantage. Yes it is an ugly business when people want this little fun game to be more than it is...Curious no?