Add the ability for a seed item to have an optional integrated JoCode in it's NBT data to produce a full tree on planting. Also include a "must plant" flag in the nbt data to tell the seed that it's voluntary planting method should never fail if it's on valid soil.
This optional data would not be used by dynamic trees itself but by world builders as a method to plant trees with scripts and command blocks by summoning seed items with the proper NBT data.

Anyone wishing to plants trees by command can now do so with the following example(post 0.8.x):

/summon Item ~ ~ ~2 {Item:{id:"dynamictrees:spruceseed",Count:1,tag:{lifespan:0,forceplant:true,code:JOJxOJxJ+v0nf1t+k06+S1+nXb1+nf1uvy7+k0+XfyWnXb1+nf0mU7ny1uvXWnWvXet1770717} }}

Syntax:
/summon Item <x> <y> <z> {Item:{id:"<seed item id>",Count:1,tag:{lifespan:0,forceplant:true,code:<JoCode>} } }