I know Growthcraft support has been planned for a while now, but, its probably possible now, the source has been released.

https://github.com/Gwafu/Growthcraft-1.7

If however you take suggestions for your support, I suggest stuff for the ferment barrel, so new forms of booze can be created. I have a few ideas of what I want to do with it, including some drinks that should have been put in Growthcraft but never got added.

Iirc the devs of the Growthcraft Community Edition want to make native support :)

Have you asked the Growcraft Developer to add Minetweaker support himself?
Because it's always easier to have that in a mod itself, than external in modtweaker.

Because the dev of Growthcraft is rather inactive.

Isn't @Gwafu back in action again? He has opened a repo and the latest commit is a day old.

http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/1286298-growthcraft-jul-15-2014-proper-1-7-10-release?comment=2081