As a warning, what comes next could be totally stupid, as I am no pro with raws.
So, after trying out everything I could think about, even building a new farm, completely above ground (I usually channel 1 level for my farms, to prevents all the small odd squares), and various other similar operations (underground farm, spawning actual seeds directly...), I decided to resort to raw checking.
Not being used to how raws work (I can guess a bit, but that's about it), I still noticed a few differences between woods that do work and feather wood.
To start us off, the raws for the rough wood:
[PLANT:TREE_SMALL]
[NAME:tree sapling][NAME_PLURAL:tree saplings][ADJ:tree]
[USE_MATERIAL_TEMPLATE:STRUCTURAL:STRUCTURAL_PLANT_TEMPLATE]
[BASIC_MAT:LOCAL_PLANT_MAT:STRUCTUAL]
[MATERIAL_REACTION_PRODUCT:LOG_MAT:PLANT_MAT:ROUGH:WOOD]
[MATERIAL_REACTION_PRODUCT:SEED_MAT:LOCAL_PLANT_MAT:SEED]
[PREFSTRING:roots]
[PREFSTRING:farmability]
[PREFSTRING:generic names]
[WET][DRY][BIOME:NOT_FREEZING]
[SPRING][SUMMER][AUTUMN][WINTER]
[GROWDUR:4032]
[USE_MATERIAL_TEMPLATE:SEED:SEED_TEMPLATE]
[MATERIAL_VALUE:2]
[EDIBLE_VERMIN]
[SEED:tree acorn:tree acorns:0:0:1:LOCAL_PLANT_MAT:SEED]
[FREQUENCY:1]
[CLUSTERSIZE:1]
[PICKED_TILE:29][DEAD_PICKED_TILE:255]
[SHRUB_TILE:29][DEAD_SHRUB_TILE:255]
[PICKED_COLOR:0:2:1]
[SHRUB_COLOR:0:2:1][DEAD_SHRUB_COLOR:6:6:0]
[SOLID_DENSITY:600]
The steel oak is pretty similar, with a few extra entries about material (value, colour...)
Now, onto the featherwood, that I can't make work on any test (even other worlds):
[PLANT:FEATHER_SMALL]
[NAME:feather sapling][NAME_PLURAL:feather saplings][ADJ:feather]
[USE_MATERIAL_TEMPLATE:STRUCTURAL:STRUCTURAL_PLANT_TEMPLATE]
[BASIC_MAT:LOCAL_PLANT_MAT:STRUCTURAL]
[MATERIAL_REACTION_PRODUCT:LOG_MAT:PLANT_MAT:FEATHER:WOOD]
[MATERIAL_REACTION_PRODUCT:SEED_MAT:LOCAL_PLANT_MAT:SEED]
[PREFSTRING:wheightlessness]
[WET][DRY][BIOME:NOT_FREEZING][GOOD]
[USE_MATERIAL_TEMPLATE:SEED:SEED_TEMPLATE]
[MATERIAL_VALUE:5] [EDIBLE_VERMIN]
[SEED:feather acorn:feather acorns:0:0:1:LOCAL_PLANT_MAT:SEED]
[SOLID_DENSITY:600]
[SPRING][SUMMER][AUTUMN][WINTER]
[GROWDUR:4032]
[FREQUENCY:1][CLUSTERSIZE:1]
[PICKED_TILE:147][DEAD_PICKED_TILE:191]
[SHRUB_TILE:147][DEAD_SHRUB_TILE:147]
[PICKED_COLOR:7:2:0]
[SHRUB_COLOR:7:2:0][DEAD_SHRUB_COLOR:0:6:1]
For better comparison, open 2 note pads (or your prefered raw text editor).
First is the [GOOD] tag, presumably to restrict to good biomes?
The growth duration/seasons are not placed at the same place (should not matter, but I'm checking every difference)
So unless the growth duration is place-sensitive (would make absolutely no sense in this case), the only thing that "might" pose problem would be the 'good'.
I removed it, generated a world, embarked as dwarves with all required to make the sawmill, built farm, built sawmill, spawned feather_small (
create-items plant feather_small), processed one, checked the plot, and it was possible to plant them.
Now, on to the weirder part.
I go to the raws, in search of other [GOOD], and I find that sunberry has the tag too, but was able to plant it without issue (it even appears in the default above ground list without access to either plant or seed).
Still having the game openned, I spawn sun berries, make wine to get seeds, and they plant just fine in a neutral biome (not GOOD).
So the GOOD tag doesn't prevent from planting, but removing it seems to have enabled me to plant them when I get them (might still need to spawn a fully grown specimen, couldn't figure out how to spawn the seed).
Note the "seems to have", because it might not be related (somehow, no idea why).
To further my theory that the biome related tags are messing the farm thing, I spawned glass_thorn_small, glumprong_small and nether_creeper as plants, transformed them to get seeds, and sure enough, none of them appear in farm plot (above or underground).
Now, the only thing that could prove this theory wrong would be to be able to plant them without removing the tag, which I could not achieve with the same method and the tag being there.
EDIT: Actually, scratch that whole theory.
It appears that removing the GOOD tag made it available at embark, so my test is not conclusive.
Now, the question remains:
Did anyone manage to "unlock" ANY plant/tree in the farms?