Seeing how you feel about it, I'd skip the registry, and just let FMIL know that they can either donate to your favorite charity, or get you Target,
Walmart, iTunes, HomeDepot, whatever gift cards to the place you want. However, don't be surprised if you still get gifts, there are plenty of folks (me included, I'd end up sending you one...) that just like sending a gift for a wedding even if they're not there.
As for the registry cards, if you do decide to register, they would normally be either for shower invitations, or to give you your FMIL to hand out to the friends who ask about it.
I would honestly think it less tacky to get them in an invitation, than to have a request for "No gifts, cash only please". Etiquette is a constantly changing scenario. Years ago no one would ever have thought of sending an invitation to a party through e-mail, but look how popular E-Vites are now!