Maybe you only need a small bit on the flex (main CPU and the DDR to prove signal integrity?), and the rest can just go on an ugly quick PCB for software validation and bringup. Stuff it with test points, current shunt points, larger parts for easy probing, add some mounting holes too fasten it to a test rig, whatever.

Then just glomp the flex down to the test PCB with some nice Samtec connectors (you can enough free samples for a small batch of test units even) or an edge connector on the stiffener.