Another link with Roman numerals?

[pfeaster]

Differentiation within clusters is also an interesting option. Though if we focus on the [i] minim cluster in Voynichese, it becomes clear that the mere presence of a swoop does not differentiate much, it's always present. Of all words with [i], 85% end the cluster in [n], 11% end it in [r]. So in most cases you add an upwards swoop from the bottom of the last minim. The differentiation occurs when you also add an upwards swoop, but connect it elsewhere?

The value of the flourish itself for disambiguation could vary depending on the quantity of minims.  If we analyze [n] as [i] closed with a swoop, and [r] as [i] closed with a different kind of swoop, then the probabilities of one or another swoop-type would be, in Currier A:

[...ai-]
>[n] as [...an] = 3.64%
>[r] as [...ar] = 38.18%

[...aii-]
>[n] as [...ain] = 59.95%
>[r] as [...air] = 28.57%

[...aiii-]
>[n] as [...aiin] = 95.30%
>[r] as [...aiir] = 2.16%

The differences in Currier B are similar but not quite as extreme.  I'm also ignoring other options, such as [l] and [m].  But for [ai-] or [aii-], the choice of [n] to close the sequence seems less of a foregone conclusion than it is for [aiii-].

For [e] clusters, the most obvious morphological equivalent of [n] is [b], which likewise becomes more probable the more "hatchmarks" there are leading up to it (e.g., [...eeeb]) -- an interesting parallel behavior, in spite of how rare [b] is.

Early modern French cursive would probably also show an uneven distribution of "ending flourishes" based on minim quantities.  For example, a cluster of three minims would almost always be [iij], corresponding to modern [m], [in], or perhaps [ui]; the Latin word "diu" would be about the only likely exception.  But [j] would still always be most common as the final glyph, regardless of the quantity of minims -- it would just be more common to varying degrees.  So this differs from the situation with [i] clusters in Voynichese, where [ar] is more common than [an], but [aiin] is more common than [aiir].  I think you're right to suggest that if there's a connection, it's one based on general principles, and not on specifics.  But my sense is that the practice of writing, for example, "iij" for "iii" arose out of conventions of alphabetic writing, rather than being unique to the writing of numbers, so those "general principles," whatever they might be, are probably to be sought there -- or maybe in the physical mechanics of writing itself, irrespective of what it's intended to represent.