I think an individual JW could fall foul of the law if they refused to serve an ex-JW but I can't see how the Organisation could be held to account.
It might be interesting if an ex-JW did try to frequent a JW-owned business and see if they are discriminated against.
I believe cofty is on the right track. If you can prove elders will dis-fellowship someone if that keeps working for or talking to a dis-fellowship'ed individual, than I believe a case could be made. Everyone knows the threat is real and yet, strangely enough, the only time I have actually heard it put in practice was with Ray Franz!