Originally Posted by Roro

!!
I've just found out where I read that:-

Jerzy Kurylowicz (1965). 'The Evolution of Grammatical Categories,' Diogenes, No.51. pp.55-71.

This is a more or less introductry paper for general purpose, but it seems interesting, although there's no mention of 'be+p.p.' construction. If you're interested, I'll quote some passages.