There is nothing in ME or in Latin to support this definition of the word. The use of the verb prop to mean 'to support' is attested by the OED from 1507 and propping only slightly earlier - this early derivation seems unlikely. Proppage, as a term in English, dates from the 19th century.
s.
1taxproppage, fee paid to lord for right to set up booths, tents etc.:
(
1280
)
se pleyntrent al Abbé ke seon gardeyn [...] avoyt lor pyers destreynt [...] pur pychage et propage en prejudice de lur franchise
8
propage (1280)