Nothing official, like 501K (non-profit) that I know of. The Allante clubs have been soliciting XLR owners to join their organization. I think some have even been renamed "Allante/XLR" clubs.
I belong to an unofficial group called the Midwest Marauder XLR Club with members from Iowa, Illinois, Indiana, Kentucky, Ohio, Wisconsin, Georgia, Florida and Canada (not really Midwest anymore as the group has grown).
We attend all the Cadillac Racing team events east of the Mississippi (We have 7 XLRs at Mid-Ohio right now for a race - Cadillac finished 2nd, 3rd, and 4th today). I attended this, Belle Isle (Detroit), and St Petersburg this year. I can't give Cadillac Marketing enough praise for their "Cadillac Corral/VIP Hospitality suites." Many in the group also attend ALMS Racing events at Sebring (Mar 13-16 2013), Road Atlanta(Oct 17-20) and Road America (Aug 16-18).
We do at least 2 road trips a year (one week long trip in Spring and one weekend trip in Fall). Our long trip last year was from Indy to the Biltmore Estate and this year was Indy to Gettysburg. The short trip is usually to see the Fall colors in Southern Indiana.
We also attend several car shows including Corvette Homecoming in Bowling Green(Jul 25-27,2013), Corvette Funfest in Effingham, IL (Sep 13-16), Carmel, IN Artomobilia (Aug 25), Belleville, Il Octoberfest and Car Show(Sep 21-22), and next year we will have a 10th Anniversary celebration of the first XLR production at Bowling Green(TBD).