Hi Retired BA/BYJP4 Inverted Spins.
Never tried a full inverted, but used to do inverted flick rolls for a bit of fun, back in the 60s before the Mk 4 rudder posts started to crack up and all flick manoeuvres were banned.
Technique was to pull to the vertical and then, at about 120kt, full forward stick and full rudder. The aeroplane departs smartly, rolling in the direction of the applied rudder.. Recovery was interesting as you have to put recovery rudder on in the direction of the perceived roll.
Don’t think it would be a good idea for modern JP owners to try.