Ur-texts, I think, are more often discussed than they are reconstituted.

And the great difficulty with them is that short of some amazing discovery - as of the actual original text itself - there's no way of verifying your conjectures.

If you have the known ur-text (a holograph of a poem by Burns, for example), it can be interesting to compare it with later versions and, in some cases, published distortions to show what errors have crpt in and how.

On the other hand, it's very difficult to go in the opposite direction.

I think the best one can do is to make an educated guess at the themes and motifs that may have been in the ur-text rather than at its precise words.

I attempted to get close to a plausible ur-text of "Rolling Down to Old Maui" on this thread:

thread.cfm?threadid=33324#3653848

With dubious success.