Well, I think I have resolution on this issue.
The Mission 77 series tweeters are leaf-sprung, but it's just the 2 screws that secure the tweeter at the back of the black housing that have a specific torque setting.
I've been told, and this may be useful to others who may need to replace an indentical tweeter on this series of speaker, that the best way to describe the tightness of the screws is hand tight, plus a further 1/4 turn. This should get it to the correct torque, or very close.
I honestly couldn't see all along how the 4 cabinet woodscrews that secure the fixture from the front could have any effect on a spring loading in the tweeter housing itself. It just wasn't logical.
The reason they state to NOT touch the 4 cabinet woodscrew fixings in the manual is because they'll never need tightening due to low vibration from the tweeter, over-tightening the screws could stress the fixture too much causing damage, and given that the screws are so close to the delicate tweeter itself, you could easily slip with the tool and land on the tweeter.
If they didn't feel the bass drivers ever need to be checked for tightening, they'd say to NOT touch these aswell, but there is less chance of accident occuring with these larger/stronger fixtures, but there is still no need to drive them home tight. Just make sure they are secure every now and again.
I've included a pic below of the tweeter, in place, with the four fixings screws.