As the title states.

The use case for this suggestion is based on having a guaranteed drop with a chance of another. For example, say I wanted to have clay blocks drop Pyrotech clumps (2-4) guaranteed, and 25% of the time have clay balls (1-2) drop.

Implementation is up to you, but I could see it being a drop strategy that, when an item is defined with no weight, guarantees the drop.

Yeah, I think this is a great idea. This use case has come up several times now and the solution isn't very intuitive or precise.

I think it would be better implemented as a separate parameter, though. Maybe something like the following.

ZenScript

import mods.dropt.Dropt;

Dropt.list("list_name")

  .add(Dropt.rule()
      .matchBlocks(["minecraft:stone"])
      .addDrop(Dropt.drop()
          .force(true) // <-- here
          .items([<minecraft:string>])
      )
  );

Json

{
  "rules": [
    {
      "match": {
        "blocks": {
          "blocks": [
            "minecraft:stone:0"
          ]
        }
      },
      "drops": [
        {
          "force": true, // <-- here
          "item": {
            "items" : [
              "minecraft:string"
            ]
          }
        }
      ]
    }
  ]
}

API

@SubscribeEvent
public void on(DroptLoadRulesEvent event) {

  List<IDroptRuleBuilder> list = new ArrayList<>();

  list.add(DroptAPI.rule()
      .matchBlocks(new String[]{
          "minecraft:stone"
      })
      .addDrops(new IDroptDropBuilder[]{
          DroptAPI.drop().force(true).items(new String[]{ // <-- here
              DroptAPI.itemString(Items.STRING)
          })
      })
  );

  ResourceLocation resourceLocation = new ResourceLocation("my_mod_id", "rule_list_name");
  int priority = 0;
  DroptAPI.registerRuleList(resourceLocation, priority, list);
}

If the rule matches, forced drops should always drop and not count toward the dropCount or be affected by the dropStrategy: UNIQUE. Forced drops should also not affect replaceStrategy: REPLACE_ITEMS_IF_SELECTED or replaceStrategy: REPLACE_ALL_IF_SELECTED because a forced item drop would just result in negating the effectiveness of the _IF_SELECTED and essentially have the same effect as just REPLACE_ITEMS and REPLACE_ALL.

This is complete and will be available in Dropt 1.17.0