POP/ROCK Peter Townsend: ''Scoop.'' (Atco Records 90063-1.) When The Who are on vacation, and one has nothing to do, why not have some fun and release a solo album? This is what Who guitarist-writer-singer Pete Townsend did, and the result is ''Scoop.'' The two-record set is a collection of studio demos, old Who tunes, and strange songs that Mr. Townsend may have felt would bolster his standing in rock music. In many ways, he pulls it off. Cuts such as ''Love Reign O'er Me'' and the Who classic ''Behind Blue Eyes'' remind us of the brilliance of Townsend's writing. One of the record's finest pieces is a piano solo intended for the movie ''Quadrophenia'' but not used, and simply titled ''Unused piano.'' Some of this album's selections actually sound better on Townsend's solo arrangements than when first released with The Who. For instance , the mid-1970s Who hit ''Squeezebox,'' which on this album has a bouncier, upbeat fun sound to it. There are, however, several short throwaways that could easily have been left off. On the whole, this is an album of diversity and enjoyment.