"Houblon (hops) with milk can be a medical drink, to help lactation"
I'm reasonably sure that what was meant in Mercer's memoir was hop shoots (not roots) being served and eaten; after being stewed in milk.
Having never heard of hop shoots as a vegetable, I find it an intriguing idea, especially since hops are grown in the vicinity. Hope to find out if some might be available so that this Waterloo "side dish?" can be tried.