OK, I've tried LootChestUPS together with modmashsplinter and modmashsplinterresources. The prototypes (units, spawners, turrets) etc. have loot, and when I placed a loot chest and went on a killing spree, I've found some Alien Ore (made by modmashsplinterresources) in the chest. For some reason, that also worked without modmashloot.
You can check the prototypes by pressing CTRL+SHIFT+E in the game. This will open the prototype browser, where you can use CTRL+F to open the search field. Enter "unit" there, and click on "entity" in the list view. Then, open the properties of any prototype and look for the "loot" field. Expand the view for more details. I see there 2 items: Alien ore will be spawned with a probability of 0.09, Alien artifacts with a probability of 0.0045.
For testing, I'd recommend you start a new game with the map setting "Loot drop when no chest placed" turned off (loot will be accumulated in the hidden list). Then go around killing enemies and use the command /LC-show-hidden-storage every once in a while, to see if any loot has accumulated. It may take a while (chances for loot are 9% and 0.45% each time you kill something), but there should be something eventually.