[edit] I didn't test any of this, so no guarantee it'll work without additional efforts
Since the "Base" drink youre using there is apple, you could modify the orange tree and others definition to add additional drink mats with DRINK_MIX_APPLE, then reference them in the material reaction product. If you do this for every plant you want to "mix" in an additional ingredient, adding the specific products you want to mix in, you can change the reaction to select anything that has a DRINK_MIX_APPLE, and it will select fruit that can be combined with apples as you've defined.
The reaction would look like this:
[REACTION:MIXED_BREW_APPLE_ALE]
[NAME:brew things into apple ale]
[BUILDING:AQUAFILTER_STILL:NONE]
[REAGENT:A:1:PLANT_GROWTH:FRUIT:PLANT_MAT:APPLE:FRUIT] --- only accepts apples
[HAS_MATERIAL_REACTION_PRODUCT:DRINK_MAT]
[UNROTTEN]
[REAGENT:B:1:PLANT_GROWTH:FRUIT:NONE:NONE:NONE]
[HAS_MATERIAL_REACTION_PRODUCT:DRINK_MIX_APPLE] --- instead of selecting oranges specifically, it now allows any plant growth that has a DRINK_MIX_APPLE reaction product defined
[UNROTTEN]
[REAGENT:plant:1:PLANT:NONE:PLANT_MAT:TOPAZIC_BLIGHT:STRUCTURAL]
[HAS_MATERIAL_REACTION_PRODUCT:DRINK_MAT]
[UNROTTEN]
[REAGENT:barrel:1:BARREL:NONE:NONE:NONE]
[EMPTY]
[FOOD_STORAGE_CONTAINER]
[PRESERVE_REAGENT]
[DOES_NOT_DETERMINE_PRODUCT_AMOUNT]
[PRODUCT:100:5:DRINK:NONE:GET_MATERIAL_FROM_REAGENT:B:DRINK_MIX_APPLE] --- now creates whatever is defined in the fruit as its product when mixed with apples.
[PRODUCT_TO_CONTAINER:barrel/pot]
[PRODUCT_DIMENSION:150]
[SKILL:BREWING]
And you'd append this to the orange tree definition within the orange fruit definition:
[MATERIAL_REACTION_PRODUCT:DRINK_MIX_APPLE:LOCAL_PLANT_MAT:DRINK_MIX_APPLE]
and this at the end of the orange tree definition so it has a drink to produce:
[USE_MATERIAL_TEMPLATE:DRINK_MIX_APPLE:PLANT_ALCOHOL_TEMPLATE]
[STATE_NAME_ADJ:ALL_SOLID:frozen orange-apple cider]
[STATE_NAME_ADJ:LIQUID:orange-apple cider]
[STATE_NAME_ADJ:GAS:boiling orange-apple cider]
[MATERIAL_VALUE:3]
[DISPLAY_COLOR:6:0:0]
[EDIBLE_RAW]
[EDIBLE_COOKED]
[PREFIX:NONE]
You could also, for specific reactions i.e. always only orange + apple, create a plant definition that never actually appears in the world with a bunch of definitions for various mixed drinks like "orange-apple cider," "blueberry pomegranate wine" etc and have the product point to that plant's ID for the desired drink. That way you'd have to create a new reaction for each drink as well as a new product on the fake drink plant, but not have to modify the plants that are used as reagents
[REACTION:MIXED_BREW_APPLE_ALE]
[NAME:brew things into apple ale]
[BUILDING:AQUAFILTER_STILL:NONE]
[REAGENT:B:1:PLANT_GROWTH:FRUIT:PLANT_MAT:APPLE:FRUIT]
[HAS_MATERIAL_REACTION_PRODUCT:DRINK_MAT]
[UNROTTEN]
[REAGENT:B:1:PLANT_GROWTH:FRUIT:PLANT_MAT:ORANGE:FRUIT]
[HAS_MATERIAL_REACTION_PRODUCT:DRINK_MAT]
[UNROTTEN]
[REAGENT:plant:1:PLANT:NONE:PLANT_MAT:TOPAZIC_BLIGHT:STRUCTURAL]
[HAS_MATERIAL_REACTION_PRODUCT:DRINK_MAT]
[UNROTTEN]
[REAGENT:barrel:1:BARREL:NONE:NONE:NONE]
[EMPTY]
[FOOD_STORAGE_CONTAINER]
[PRESERVE_REAGENT]
[DOES_NOT_DETERMINE_PRODUCT_AMOUNT]
[PRODUCT:100:5:DRINK:NONE:PLANT_MAT:(mixed-drink plant ID):DRINK_ORANGE_APPLE]
[PRODUCT_TO_CONTAINER:barrel/pot]
[PRODUCT_DIMENSION:150]
[SKILL:BREWING]
[MATERIAL_REACTION_PRODUCT:DRINK_ORANGE_APPLE:LOCAL_PLANT_MAT:DRINK_ORANGE_APPLE]
[USE_MATERIAL_TEMPLATE:DRINK_ORANGE_APPLE:PLANT_ALCOHOL_TEMPLATE]
[STATE_NAME_ADJ:ALL_SOLID:frozen orange-apple cider]
[STATE_NAME_ADJ:LIQUID:orange-apple cider]
[STATE_NAME_ADJ:GAS:boiling orange-apple cider]
[MATERIAL_VALUE:3]
[DISPLAY_COLOR:6:0:0]
[EDIBLE_RAW]
[EDIBLE_COOKED]
[PREFIX:NONE]