That's really kind of funny... 
This thread brought to my attention that Serif already uses a system to toggle between two tools – generally a super helpful feature – I even opened a feature request for this.
Serif just happened to implement that tool-toggle a way that it either gets terribly in the way or doesn't get discovered at all. I dare calling the current implementation plain wrong – it doesn't do anything good and can not even get turned off – it really should get fixed asap.

What I am used to from  quite a variety of programs (including Photoshop) is that one may temporarily call another tool by pressing and holding down a letter key. That way one may lay down a couple of brush-strokes while actually drawing a path: Press (and release) P to call the path tool and click to lay down anchor points. In order to access to the brush tool one would press (and hold!) the letter B. As soon as you let go the B-key one reverts to the Pen tool exactly in the position where one had left off – one may continue laying down points in that path. The greatest advantage of this implementation is that it can not get called by accident. There's simply no other reason to press and keep holding a (Non Modifier-Key) right now = it can not get in the way. The second great advantage is that this sort of toogle is broadly known and expected behaviour. Hooking this up should not conflict with anything, as you don't use this event yet (normal keys held down).