Hi Geoff
I came across this from the University of North Carolina Department of Music (2013).
The earliest documentation of a lap steel with pedal device is a patent applied for in 1936 by Antony P. Freeman of San Francisco. Granted to Freeman in 1938. U.S. patent no. 2,122,396 shows a lap steel of Rickenbacker's "frying pan" model mounted on a stand and operated y two pedals. The patent states that two prototypes were played by "outstanding guitarists".
It is part of much larger document on the pedal steel guitar, about 240 pages. It contains examples of music of top players written in standard notation (not tab) with position notes similar to that used by clasical guitarists. If you were interested and can't find it, I could send you the pdf file.
Billy


