If a custom potion is created using datapacks or commands, placing redstone or glowstone into the ingredient slot will do nothing (I assume this is expected behavior since data-driven brewing recipes have yet to be added).
However, placing gunpowder into the ingredient slot will actually consume the gunpowder and play the brewing animation as if it were converting the custom potions into splash potions; only to then do nothing when the brewing process completes, consuming the gunpowder and leaving the custom potion unchanged.

Steps to reproduce:

1. Create a world with the attached datapack.
2. On 1.21-pre3 and above, run /loot give @s loot minecraft:archaeology/desert_pyramid a couple of times until you get the Potion of Embalming. On 1.20.6, run /loot give @s loot minecraft:potion_of_embalming.
3. Put it in a brewing stand alongside with gunpowder and blaze powder.
4. Wait until the brewing stand finishes brewing.

Observed result:
Nothing happens to the potion.

Expected result:
The potion becomes a splash potion.