Could you fit coilovers on the rear of the e31?

In all honesty I would say no. There would have to be major modifications to the turret tops and recess as they are already renowned for failing. With strengthener plates being available to spread the load on the top, putting more stress there is only asking for trouble.....

IMHO...

Schnitz's IMHO is spot-on.

Without major strengthening of the rear turrets, probably with the addition of a full roll cage, the system would just not last.

Additionally, there is insufficient room for the spring around the OS strut without moving the fuel filler pipe.

In my opinion, the upper mount would need proper chassis rails inside the trunk to take the load, the pressed steel shell is too weak.

I have discussed this numerous times and here is a pic clearly showing how a coil-over strut spring would foul against the fuel filler........and this is why in your pic of the coilover, there is a very temporary undersized plastic hose being used to fill the car with fuel.