While having a reading another blog, I thought this was interesting, from the Maelstrom's Edge blog.
The article is about adding green stuff gas masks to figures (SF rather than historical) but the writer uses tools he refers to as "silicon clay shapers" or "colour shapers".
Now my local The Works (suppliers of cheap books, hobby supplies and tat) offer something which looks just the ticket.
Looking like silicon rubber paint brushes, it is claimed they do not stick to green stuff and hence can be used to sculpt green stuff before it sets.
We shall see.