I think Paul Cockshott is on the right track in arguing that the
commodity form is more general than capitalist production, and that the
concept of abstract labor, like money, is an effect of the commodity
form. I sometimes phrase this as "to the extent that production takes the
commodity form, money (or abstract labor) tends to emerge."
But for the same reasons it seems off the track to view feudal
exploitation as taking the form of the expropriation of surplus labor,
rather than as the appropriation of a surplus product in concrete form.
(Just as in ancient societies there was commodity production and even
nascent capitalist production in feudal societies alongside feudal
production.)
I would be cautious about the concept of the total labor time of society,
since every society has a number of different modes of production and
forms of labor and product coexisting. Marx often implicitly argues as if
capitalist society were made up entirely of capitalist producers, but
that isn't true; for example, noncommodity labor plays an important part
in reproducing the labor force in capitalist society.
Duncan