in my recipes.zs i have
val levitationPotion = <minecraft:potion>.withTag({Potion:"crafttweaker:levitation", CustomPotionEffects: [{Ambient: 0 as byte, CurativeItems: [{id: "minecraft:milk_bucket", Count: 1 as byte, Damage: 0 as short}], ShowParticles: 1 as byte, Duration: 200, Id: 25 as byte, Amplifier: 0 as byte}]});
which i can set as the output from a normal craft, and gives the expected 10 seconds of levitation 1. in extra_alchemy_recipes.cfg i have
minecraft:awkward & nex:food_meat_ghast_cooked = crafttweaker:levitation
which doesn't even allow me to insert the ghast meat into the stand's input. if i replace "crafttweaker:levitation" with "enderzoo:floating" i can insert the ghast meat, and brew the potion from the enderzoo mod.
what nbt tags etc, do i need to set to allow extra alchemy to "see" my custom potion and set a brewing recipe for it, please?

whatever i name the potion in .withTag({Potion:""}), it doesn't show up in your log, or in /tellme's list of potiontypes. it still has the tag set correctly in game, the potion itself works, but it's not showing up as a potiontype. it doesn't help to use itemUtils.createPotion(), either.
is there a registry of potiontypes? and if there is, and that's what you're checking, how would i add my custom potion to it? i don't see anything about this in the crafttweaker docs :/

The problem here is how you are creating the potion. Using the withTag function you are creating a potion that has a custom effect, instead of adding a new PotionType into the registry (which is the only way my mod can load new recipes). I quickly read through the CraftTweaker page to see if there was a way to add them, but it doesn't look like it.
I suggest going to their github page and opening a new issue requesting the option, since non-potionType custom recipes are out of this mod's scope

okies, thanks anyway!