The Modern Forgery Hypothesis

[asteckley]

The purpose of the hypothesis would be to determine if it was the Voynich, or another manuscript entirely, and then describe what it could have looked like, and maybe help in finding it, if it still exists.

I can't resist...

Given that you don't accept the text in the Barschius letter to be an adequate description of the Voynich MS, finding one that does fit seems like an impossible task.

Clever point.  But it doesn't really follow, does it. 
Being an inadequate description of one document does not mean that it cannot be a more adequate description of some other document.

Furthermore the letter might be describing the other document with equal inadequacy -- which would have a bearing on the strength of connection  to the VMS (since the two documents would become tied for the honor.) Or the other document could come along with additional evidence securing a superior connection to the letter (e.g. suppose it had an inscription by Barschius himself echoing the same description.)