This might already be known, but I don't think I've seen it before:
Entities determine what they have access to during world gen. They will build instances of any toys in their entity definition at this time. Because we have access to the save raws now, you can alter the entity's definition after world gen. This means that you can let a parent civ build items as toys during world gen, then change it afterward so that the items are only available to the player as rxn products, or not at all. It should be possible to do the same thing with instruments and with weapons and armor.
World gen:
[TOY:ITEM_TOY_BRAIN_IN_JAR_ELF]
[TOY:ITEM_TOY_BRAIN_IN_JAR_HUMAN]
[TOY:ITEM_TOY_BRAIN_IN_JAR_DWARF]
[TOY:ITEM_TOY_CASKET_OF_BILESTONE]
[TOY:ITEM_TOY_SPATIAL_WARP_SEA]
[TOY:ITEM_TOY_SPATIAL_WARP_SKY]
[TOY:ITEM_TOY_SPATIAL_WARP_FOREST]
[TOY:ITEM_TOY_SPATIAL_WARP_FIELD]
[TOY:ITEM_TOY_SPATIAL_WARP_CAVE]
[TOY:ITEM_TOY_SPATIAL_WARP_DEEPS]
[TOY:ITEM_TOY_MOTHER_STONE]
etc...
After world gen:
TOY:ITEM_TOY_BRAIN_IN_JAR_ELF
TOY:ITEM_TOY_BRAIN_IN_JAR_HUMAN
TOY:ITEM_TOY_BRAIN_IN_JAR_DWARF
TOY:ITEM_TOY_CASKET_OF_BILESTONE
TOY:ITEM_TOY_SPATIAL_WARP_SEA
TOY:ITEM_TOY_SPATIAL_WARP_SKY
TOY:ITEM_TOY_SPATIAL_WARP_FOREST
TOY:ITEM_TOY_SPATIAL_WARP_FIELD
TOY:ITEM_TOY_SPATIAL_WARP_CAVE
TOY:ITEM_TOY_SPATIAL_WARP_DEEPS
TOY:ITEM_TOY_MOTHER_STONE
etc
The above tokens are from my illithid mod. I have embarked several times; you can definitely buy the stuff at embark. I just had my first Illithid caravan, and they had one of the items as well (tiny caravan, three mules). I have also tested to make sure my craftsmen don't make these toys despite them not appearing in the raw. As far as I've seen, they don't. I did have one mysterious fire on embark, but it didn't happen before or since, so I'm chalking it up to some other bug, possibly with bonfire materials? Who knows. Anyway, this should be useful to people doing Alchemy mods.