I tried both ways and I have success.
Using scamtank's method I have this
[REACTION:MAKE_WAX_CRAFTS]
[NAME:Burn wooden _ scamtank]
[BUILDING:KILN:CUSTOM_S]
[REAGENT:object:1:NONE:NONE:NONE:NONE]
[ANY_PLANT_MATERIAL]
[HARD_ITEM_MATERIAL]
[PRODUCT:100:1:BAR:NO_SUBTYPE:COAL:CHARCOAL][PRODUCT_DIMENSION:150]
[SKILL:SMELT]
Edit: Use the following reaction instead or your burner will take barrels full of booze (or whatnot) and turn them to charcoal. I would recommend that you manage this reaction with a stockpile anyway to prevent burning masterworks. It will burn any wooden object to make one bar of charcoal.
[REACTION:MAKE_WAX_CRAFTS]
[NAME:Burn wooden _ scamtank]
[BUILDING:KILN:CUSTOM_S]
[REAGENT:object:1:NONE:NONE:NONE:NONE]
[ANY_PLANT_MATERIAL]
[HARD_ITEM_MATERIAL]
[EMPTY]
[PRODUCT:100:1:BAR:NO_SUBTYPE:COAL:CHARCOAL][PRODUCT_DIMENSION:150]
[SKILL:SMELT]
This works exactly as intended. When I used the method suggested by Quietust
[REACTION:PRESS_HONEYCOMB]
[NAME:Burn wooden object _ Quietust]
[BUILDING:KILN:CUSTOM_Q]
[REAGENT:object:1:NONE:NONE:NONE:NONE]
[HAS_REACTION_CLASS:WOOD]
[PRODUCT:100:1:BAR:NO_SUBTYPE:COAL:CHARCOAL][PRODUCT_DIMENSION:150]
[SKILL:SMELT]
[MATERIAL_TEMPLATE:WOOD_TEMPLATE]
...
[REACTION_CLASS:WOOD] (last line in template)
I get a weird effect. The first round of reaction goes exactly as intended. However, on the subsequent reactions my worker proceeds to grab the existing bar of charcoal in the kiln and make hundreds of bars of charcoal from it, very odd. (I think he might be making exactly 150, due to the material size?) I'm sure I've screwed something up.
Now I'm trying to make the other reaction I mentioned above - burning cloth to ash. I tried this
[REACTION:PRESS_HONEYCOMB]
[NAME:Burn cloth _ Ygvir]
[BUILDING:KILN:CUSTOM_Y]
[REAGENT:object:1:NONE:NONE:NONE:NONE]
[ANY_PLANT_MATERIAL]
[SOFT_ITEM_MATERIAL]
[PRODUCT:100:1:BAR:NONE:ASH:NONE][PRODUCT_DIMENSION:150]
[SKILL:SMELT]
But it still seems to grab any wooden object and make ash... Maybe I have the wrong token for soft items?
Oh, one more thing... is there any way to restrict this to items of a certain size?
Thanks for all the help!