ON THE TOWN: There are billboards and then there are billboards-- and we have to admit that David Lee Roth has somehow set a new standard in promotional wizardry with his Live Billboard, which will be unveiled Thursday at 5:30 p.m. on the roof of Tower Records' Sunset Boulevard flagship store. The cover of Roth's new "Skyscraper" album spotlights him scaling Half Dome in Yosemite. So the billboard touting his record has been fashioned in the shape of a mountain--and populated with several real young lovelies, dressed as mountain climbers, who will be stationed on the mountain 24 hours a day through the weekend. If they make it through a Friday night on Sunset Strip, we think they deserve triple-scale combat pay. (And say what you will about the alpine gear, but that's more clothes than Roth's female supporting cast has worn in any previous project.) . . . Good news on the local front: Joe Ely will be in town Feb. 5, playing a rare club double-header. The Texas honky-tonk guitarist will do a solo acoustic set at 8 p.m. at McCabe's, then move on to the Music Machine, where his band will headline for Rosie Flores and ex-Long Ryder Tom Stevens. . . . KCSN-FM gets its KCSN Barn Dance off on the right foot with a live remote Tuesday at 9 p.m. from the Valley's Little Nashville club (13350 Sherman Way). Hosting the festivities will be Ronnie Mack, who'll be joined by Dave Alvin, Rosie Flores, Billy Swan, James Intveld and Ray Campi (they'll all be playing at the club till the wee hours if you want to stop by). . . . The only sour note: The Long Ryders have called it quits, despite making several excellent albums over the past few years. In addition to Tom Stevens starting his own band, leader Sid Griffin and drummer Greg Sowders are planning to carry on with a new lineup later this spring.