Other way around: When Glenn went AWOL to the Purps, Mel become the lead singer for a few years, having to replicate Glenn's voice on the older songs. The ZZ Top inspiration was never sung by Glenn, it's from the first album (1974) without him where Mel has assumed lead vocal duties. The live version features Pete Goalby (later with Uriah Heep and at one point almost with Rainbow), the only (late) phase where Trapeze featured a lead vocalist who did not double on bass or guitar.
Another almost-famous-band: The criminally underrated Legs Diamond, often referred to as "the American Deep Purple" though I never found that comparison apt:
Cracker of a song!
PS: Ok, I now hear that the DP-comparison is not entirely baseless ...
Jon Lord could have sued!