But in my brief attempt to answer my own question I only found pages detailing pico balloon launches from the UK.
In reality, it's an almost unenforcable law to begin with. Most of the WSPR transmissions are on the order of 10mW (probably similar to the Bluetooth output of your cell phone). It's not unusual for these transmissions to be heard several thousand miles away, which is quite amazing!
I know for other amateur balloon experiments such as weather balloons, they often use the ISM bands instead in the UK.