I don't know how sincere he was but when I spoke to Mark (after years of trying) at his Cannes market company booth, he said he wanted to release a SILT special edition including director's cut and that he would talk WB into it (which was about a year before the Blu-ray was released), so yeah he must have kept a Beta tape copy or something...
But being inside the corporate distribution business, he would probably never get it out there without permission from WB.
The only chance there I can see now, and because WB seems to have changed their third-party licensing policy in the meantime, is if say a German indie label would sublicense the movie, and get permission from both Lester and WB to add the extended cut as an extra (which is not always to the liking of big studios either so there's some lobbying to do if you don't want to ruin your relationship with them to acquire future title).
His generation soon or late will be gone and I am afraid that nobody will see their masterpieces hidden somewhere in vaults .... and this is sad
Indeed...!