Hi,
I'm glad to know you've figured it out!
1. Can these extra outs be mapped to an attribute like a patched OSC out message?
Extra osc outputs can only be sent with the osc.send function.
2. If not, can the default prefix for mapped OSC outputs be changed to a custom message?
Yes, you specify the full address when configuring the extra OSC outputs. The prefix is only used for the inputs.
A note about the osc.send function. It only send a OSC message when one of the value to be sent actually changes. So if you have osc.send(0, 100), the value 100 will only be sent once, even if you deactivate and reactivate the grid.