My research on several twentieth century composers has turned up numerous toccatas, since most of these guys are huge Bach/baroque enthusiasts. Some of them aren't solo piano pieces, but oh well.
Arnold Bax - Toccata in Db
Niels Viggo Bentzon - Toccata, op. 10 (1941)
Arthur Honegger - Toccata and Variations (1916)
Alexandre Tansman - Prelude & Toccata (1941), 3rd movement of his 3rd sonata, 3rd movement of his 1st sonatine, 6th movement of the Suite dans le style ancien, and Hommage à Arthur Rubinstein, 2nd movement. As well, lots of movements in his chamber works are toccatas.
Marcel Mihalovici - Toccata pour piano et orchestre, op. 44 (1950)
Pierre Sancan - Toccata (1943)
Roy Agnew - Toccata Tragica
Eduard Destenay - Toccata in a minor, op. 45
Jean Absil - 5th piece in 5 Bagatelles, op. 27, 4th movement of Grande Suite, op. 62, last section of Variations, op. 93. As with Tansman, a lot of toccata sections show up in his chamber work.
Emma Lou Diemer - Toccata
G. Tsouyopoulos - Toccata III (I haven't heard the other two, if they exist...)
Henk Badings - Toccata for marimba (1973)
Emmy Frensel Wegener - Toccata (1938?)
Edison Denisov - 1st movement of his Sonata for guitar (1981)
Karel Mengelberg - Toccata (written in dedication to Willem Pijper, 1950)
Alun Hoddinott - movements in his 4th Sonata (1968) and 5th sonata (1969), likely other works
William Mathias - Toccata alla danza (1965)
Roy Harris - Toccata (1950)