Yep. I spent some time working with NASA's spin tunnel (with free spinning models and rotary balance) so I have seen lots of data.
Somewhere online is a video of NASA flight tests of a Grumman Trainer which spins extremely flat with no power - and is generally unrecoverable.
NASA also has lots of model spin tunnel data on the same aeroplane and you should be able to find that on the NASA Langley technical data server - you may also find a copy of the same stuff at Cranfield Uni's online library.
Regardless of how you want to define lift or drag, and I agree it is non-linear, you will find that the resultant aerodynamic force acting vertically upwards will generally continue to increase (post-stall) until approx 90 deg AoA.
Depending on the balance of all the aerodynamic and inertia forces and moments - driven by controls and power as appropriate - will determine how flat a specific aeroplane will spin.
I should have mentioned Reynolds Number - you will find a NASA report on that subject too wrt comparison of model spin tests with full scale spin tests.
Some of us also played around with miniature spin tunnels and very small but dynamically scaled models which also had reasonable correlation with full scale. At one time there was one in NASA Langley's Visitor Centre with a Grumman Trainer happily spinning quite flat.