Were guerilla and Fabian tactics feasible in the medieval period? I know the Romans employed the concept (hence Fabian tactics), but it seems like most nobles would be a little averse to it given that it requires more decentralized leadership and a more empowered populace to maintain supplies, morale, equipment, etc.

Absolutely. 

If you look at the history of the various wars between Wales and Norman England, leading up to Edward I’s conquest, the Welsh enjoyed their biggest successes when they used guerrilla tactics against the better-equipped but slower English armies, using a combination of the longbow, hit-and-run tactics, and the Welsh mountains as secure refuges. 

Likewise, Bertrand du Guesclin, Marshal of France from 1370-1380, took over after the disasters of Crecy and Poiters, and used Fabian tactics to great effect in Brittany, Castille, and Poitiers, reversing most of the French losses from that phase of the Hundred Years War.