Well, turns out I’m not the first person who thought that the locust plague of Joel 1 might actually refer to an invading army. Not only did I find Sweeney making a similar connection in his commentary, I found this quote from the Syriac father, Isho’dad of Merv (9th century AD):
The mashota (“cutting locust”) is similar to a larva. It is black and longer than a larva; when it falls to the ground, it does not destroy completely the plant but devours just the leaves and does not touch the rest. Through it the prophet signifies Tiglath-pileser, because the troubles that he caused to the people of Israel were mild. Her calls Shalmaneser the flying locust, because the destruction that he caused was more serious than than by Tiglath-pileser. He calls zahla the crawling locust, which does not fly and feeds on everything. He signifies through it Sennacherib, because he surpasses his predecessor in the ruin caused and brings about the annihilation of the ten tribes. The sarsoura creeps on the ground and is only equipped with a string; when it strikes the roots of a tree, any tree it finds, it immediately withers. And he signifies through it Nebuchadnezzar, the cause of total destruction. He calls vines the common people, fig trees the important persons, whom that Assyrians and Babylonians deported in captivity.
Isho’dad of Merv, cited in Ferrerio and Oden, eds., The Twelve Prophets, vol. 14 of The Ancient Christian Commentary on Scripture, 60.
While his detail is a bit fanciful, it is interesting nonetheless that this ancient father also thought that the locusts may simply be a metaphor for the destruction caused by the Assyrian or Babylonian war machine.