/ic/ - Artwork/Critique

I'm not sure where else to ask this, in CSP is there a way to make the lasso fill tool refer to the lineart ref layer? The closest I've gotten is by clipping layers but depending on the shape it won't work.

I attached a picture of what I'm talking about, I want to get the result on the right using the red stroke on the left. I can get the result I want by drawing the line then using the paint bucket (refering to the ref layer) but that's 2 buttons instead of 1.