Believe it or else, the ABC Afterschool Special did spawn a spin-off.
The Time For Timer interstitals began after the microscopic fellow made his debut in 1973's The Incredible, Idelible, Magical, Physical Mystery Trip, and a sequel less than a year later, The Magical Mystery Trip Through Little Red's Head.
You might say the Farrelly brothers used this as inspiration for their movie, "Osmosis Jones", which in turn led to a Saturday spin-off of its own, Ozzy & Drix, several years later. On this Mystery Trip, Timer (Len Maxwell, ex-Batfink) leads a pair of youngsters on a guided tour of their uncle. In the live-action portion, Hal Smith (Davey & Goliath, ex-The Andy Griffith Show) plays Uncle Carl, while Kim Richards (ex-Nanny & The Professor) plays his niece. However, they used other actors when the kids turned animated. Don't ask. Let's scope it out.
For the sequel, another Len (Weinrib, that is) took over the role of Timer. Better singer, I'll grant, and the role would be his for the remainder of Timer's appearances. Maxwell, meanwhile, dropped out of sight, or so it seemed, save maybe for some commercial work, until MTV hired him for Celebrity Deathmatch in the 90's. Maxwell acquits himself well as a singer, too, as you can tell.
Aside from the opening & closing to the 1st season of The Pink Panther Show (1969), DePatie-Freleng didn't do much for live-action, and this would be the end of their live-action output. The sequel, which they also did, was fully animated.