Naw, it crossed my mind too, but Roger only discovered the TBird during the sessions for Rainbow's Down to Earth in 1979:
And Hair of the Dog (1975) was actually the first album Nazareth recorded without Roger (Manny Charlton produced it) after he had produced three Naz albums in sequence and Naz began finding their feet in the studio.
But you never know - there were always friendly relations between Nazareth and the DP camp, maybe Agnew's bass ended up with Roger? Roger produced and played bass (this time for real) on Dan McCafferty's first solo album in 1975, but I'm not passing judgement on what type of bass Roger played here (but his mannerisms are obvious):