Er, mr blobby, I’d love to know why you say it wouldn’t? I have run many bikes with mismatched shifters and mechs. The mech doesn’t know how far it is allowed to go between stops, only how far it is allowed to go in its entire range.
I have two bikes in the cellar like this right now. One with 10 spd dura ace bar end shifters running an 8 spd xtr mech on a 9 spd sram casette. The other has 9speed tiagra shifters running a 7spd 105 mech over an 8of9on7 cassette (google this if your no sure what it is – it’s a sheldon brown trick)
The bottom line Jordie is that if your mech has the same or more speeds than your shifter it will work. In some cases, like I have outlined you can get the mech to stretch a little bit further than shimano intended!