I like the idea of a WW jersey so here are two cents:
First, do short sleeve jerseys only. I have tons of team kit and am picky about shorts but I do pick up interesting jerseys from time to time. Doubtful that I am flying solo on that. I also enjoy this forum a great deal and would wear a WW jersey without hesitation.
At risk of recapping the obvious -- To make this work someone needs to put in effort to form a design. That is the easy part. The hard part is someone needs to take the risk to print the jerseys and that has a large upfront cost of a couple thousand dollars. That "someone" likely also needs to be a known member of the group so that folks will send said person money for a jersey with expectation of receiving said jersey. This lucky someone gets blessed with processing orders and mailing product. As such, that someone also needs to be a bit altruistic because this is a PIA unless you are already set up to do it (see below).
Having done a few things like this with internet groups I would estimate that if 50 folks said they want a jersey then 15-20 will pony up the cash for one early and then another 10-15 folks will come around a few months later to ask if the shirts are still available. Take home is if someone makes the investment in printing 50 shirts it might take a year but they'll eventually all sell.
A solution to the problem is obvious - a known WW with a known and trusted internet bike business makes the shirts and sells them (at a small profit of course). Or perhaps a business that caters to Weight Weenies and derives some benefit from posting to the site could step up. I would suggest that such an organization be allowed to put their logo on the shirt sleeves and obtain some relatively low cost targetted advertising
My suggestion for design would be a simple logo on a black jeresy (black is slimming and one can never be too slim). I would do a big orange and white Weight Weenie ANT mascot on the front and back of the jersey and "Weight Weenies" in simple block font on the side panels. I recall some issue about the ANT being protected but it is a much loved mascot and perhaps the owner or logo originator could bless the project.
Hope that was entertaining - I'd take two medium ANT jerseys and might be willing to actually help with the project if it gains traction.