A Numeric Solution

[Jorge_Stolfi]

simply substituting ascending numbers for letters of the alphabet (A = 1, B = 2, L = 10, M = 11, etc.)

Well, if "a" means 1, and 1 means "k", then "a" means "k".  That would be a simple alphabetic substitution cipher.  And those were already excluded...

(... if the underlying language is assumed to be European ...)

or encipherment via Polybius Square (A = 1-1, B = 1-2, F = 2-1, etc.)

That would leave the words twice as long as those of the original language.  But one vexing feature of Voynichese is that its words are shorter than normal...

All the best, --stolfi