The earliest known report of a performer freeing himself from a straitjacket while hanging upside-down appeared in the Yonkers Statesman of Dec. 6, 1912, done the previous day at the Orpheum Theater in New York City by Al Pitroff, performing under the name of “Mysterio” (reference discovered by Bill Mullins).
In an advertisement in 1920, British performer Madge Belmont (Mary Jane Elizabeth Hawkins, 1877-1931) made a claim of having done this feat in 1912, but no evidence of such a performance has been discovered. Indeed, the first reports of escape performances by Belmont are for the autumn of 1913, with a possibility of escape performances by her as early as June of that year.
Houdini made this feat iconic, but did not perform it until 1915.
For further details, see “Madge Belmont: America's Escape Artist & Straitjacket Expert” by Gary Hunt in Magicol, No. 194, Sep. 2019, p. 43.