Hi there,
I use a Morley volume pedal to adjust between arco and pizz. Because my pickup (a K&K Double Bass Max) reads teh arco signal as louder than the pizz signal, I set the Morley so that the minimum volume, ie for arco - is when my heel is down, and then maximum volume is when my toe is down.
Another alternative I've thought of (but have yet to try) is to use an Ernie Ball Stereo/Blend Volume pedal, where you can blend your incoming signal between two different outputs. The practical use would be to have one output eq'd for pizz and the other for arco.
I like this, and my current set-up as it's all done without clicking or stamping on Boss-style pedals. Who wants to do that several times in a song?
Good luck.