Thomas Chalmers undoubtedly cribbed his "disposable population" from Turgot's classe disponible. Turgot's meaning seems to be different from Chalmers's. Turgot uses the term to refer to the class's revenue coming from a surplus of produce and thus being available for use however the proprietor wishes. That is the revenue could be used for luxury consumption or it could be used for improvement of lands, purchase of machines, etc. I would take Turgot's classe disponible to be roughly equivalent to rentier.
The disposability of Chalmers's disposable population, on the other hand, has to do with the facility with which they could be reassigned to different occupations -- such as the military. That latter usage brings to mind that "disposable forces" was a term of military strategy that referred to military units that could be quickly moved to a new location in response to an enemy threat. The term appears to have been most widely in use in the 19th century.