Since I didn't know much about when mushrooms were first cultivated, I used my google fu to find out. Although they were cultivated in Asia much earlier (around 600AD, apparently) they weren't cultivated in Europe until the mid 17th century. So maybe this explains why mushrooms just don't show up a whole lot on extant menus, they really may have been a limited resource. Once I've finished checking menu sources, I'll have a quick look through what the Italians that were writing about food had to say about mushrooms, and also check Gerrard's herbal out.
The three sites I visited were:
I also just realised that by default I'll always check at least three sources, similarly to getting three quotes for plumbing or some such before deciding on a price. Incidentally, I also checked Wikipedia but in this case, unusually I was unable to find the information there.