There is a workaround
You can modify the digging in battle. Simply reload a number of times and check for chests after a unit has moved - this gives you 5 different digouts per turn, provided you use 5 troop army. Surprisingly, this produces some wondrous results. For example, I'm halfway through Scarlet with the Warrior and managed to dig 3 depth of thought, 1 Song of the Wind (hello hayterrants!), 1 Call Death (dug in the training fights - hello Mr. Bone Dragon), 2 Titan's Sword, 1 Titan's Armor, 2 Flaming Eyes and 1 Call Collosus (on Debir, unused yet because I have only 1300 leadership). The rest of the fights I settled for a rune of a random kind (with preference to mind and magic ones).
I know it's a lot of restarting, but if you want to squeeze out the most of a game, one must compromise