Alright, so I've trialed Sewline's fabric pens and pencil on needle felted wool. That is, wool roving that has been felted already, not the pre-felted floofy wool!
I was hoping to use the pens to outline difficult patterns that required symmetry, then I could erase them once I filled in the pattern with needle felting. The plan was for patterns like tiger stripes, where you need to be really careful with keeping the 'white space' between the patterns to be consistent and not touching each other. It's impossible sometimes to gauge or measure it by the naked eye.
It sadly didn't work because the wool surface isn't 'smooth' like paper or regular fabric. The wool didn't pick up the pen ink or lead.
What I had tried were these ones; they do work brilliantly however for their intended purpose on fabric!