That AAIB report should be required reading for every Yak pilot. I, too, used to think that the 'belly slap' check would reveal dropped articles in the tail area. Sadly not - indeed the design seems ideal for picking up such debris in the elevator mechanism during up vertical manouvres, with subsequent jamming of the elevator in the attempted pull out from the down vertical as in this tragic instance..
There is reference to fitting of a ceconite bulkhead ahead of the elevator mechanism in the tail to prevent such occurances. But as the mention of the Russian accident shows, where a camera lens fouled the under-seat elevator linkage, that won't be a complete cure.
SSD