Well I have done a market wallet that way to serve as a container for small stuff in my blanket rolled up for a tumpline. I baked the item in the oven on the lowest setting which was 170 degrees, for about ten minutes, then dipped the hot, dry cloth in melted beeswax using a pair of tongs. This kept the amount of wax kept by the item to a minimum.
Now as for a haversack, well I would go with boiled linseed oil and a three week cure, as the wax will get tacky in hot weather and rub off on your clothes. The boiled linseed oil when properly cured seems to have much less of a problem.
LD