I don't know when they patched this bug, but it looks like Pokemon can now hold 'non-held' items, including things like Cobwebs and Leaves. I discovered this because of some vestigial drops I'd left in (for Spinarak, specifically), and found I was able to steal the cobwebs. However, when I tried to tweak Cherubi to hold pink cherry leaves, i was unable to steal them...

...Until I rebooted. I don't know how, but it appears when the Minecraft client starts, it is capable of putting non-held items into Pokemon's hands. It's worth taking note of this, to improve the thief skill.