Welcome to the forums!

When playing with my band, I would like to gently boost the instruments volume when playing solos or instrumental parts.

Since the master volume knob can be used to increase the volume can we assume you are wanting a variable control that can be used hands-free?

I've just bought the FC7 expression pedal, and have used the spring adjustment to define the 10% limit point, after which the boost should be applied.

The adjustment on that pedal just controls the reset position. The pedal is ALWAYS going to send a value from 0-127 depending on the pedal position. It is NOT going to send a value of 0 for the first 10% of travel.

The pedal/pot doesn't use a linear curve but if it did then setting the adjustment to 10% means the pedal would return to a position of 13 (10% of 127).

I would like to configure my CP76 such that the pedal has no effect when it is below the spring point, and boost the volume when it is pushed all the way forward beyond this point.

The best way to do that is to use an external inline pedal like the Boss FV-500L. Being in the signal path AFTER the instrument means it will work the same way for every performance.

Since I already have a lot of Live Sets configured, I would like this behaviour to apply across these Live Sets. From what I have read, it seems the volume boost is Live Set specific?

Yes - you would need to alter every Live Set.

A simpler option would be to use a simple on/off footswitch, but this also seems not to be possible. 

Not sure what you mean by 'not to be possible'. Different pedals work differently. Some pedals (not Yamaha) are pure ON/OFF - they either send 0 or 127 and can often be configured for polarity.

Yamaha pedals typically send 0-127 and it is up to the instrument to decide what values should be used and how to use them. That often involves a control curve which is typically non-linear.

Try the Boss pedal

Many thanks Pete,

The Boss FV-500L is what I need. Great informative reply, very helpful.