You're backpacks added via the Forestry API are not being correctly registered to FML. You need to take the item that the Forestry API returns and do GameRegistry.registerItem() on it.

See ForestryMC/ForestryMC#244

Umm. I am doing that :S

https://github.com/joshiejack/Mariculture/blob/master/src/main/java/joshie/mariculture/plugins/PluginForestry.java

Ah, I was wrong, that actually looks like an outdated API now that I look closer.

Yeah looks like it. Last time I updated it was in June.