On Sun, 29 Jan 2012 16:54:15 +0100, Arnt Richard Johansen <[log in to unmask]> wrote:

>Is such a system plausible in the sense that it could have evolved

I think nearly all of it is plausible as a synchronic state of affairs.  But

>* The two remaining consonants, the liquids /l L/, are unaffected by the
mutation system. They might need different strategies for singularization
(classifier nouns, for instance), or they may be unsingularizable altogether.

You do mean they're not in the mutation system at all, not just that /l/
mutates to /l/ and /L/ to /L/, right?  In that case, the conclusion that I
think is nigh forced is that at some point in history, there were _no_ nouns
beginning with /l L/  (or virtually none).  If there were once some, they
must've either been sound- or morphologically-changed away before the onset
of this system, or else originally included in the system and then
analogised out of existence. 

Recent borrowings with initial /l L/ I suppose could be handled in one of
the two ways you mention, with a genuine gap in the paradigm, but that only
feels likely to me if either 
- singularisation is more derivational that inflectional (AIUI these kind of
systemic blockings are characteristic of derivation: e.g. the English
deadjectival inchoative in -en, which selects obstruents); or
- they are still extremely infrequent.

If singularisation is inflectional and /l L/ stems aren't freaks, I'd expect
them just to have identical plural and singular.  Or, if that isn't to
happen and you do posit a classifier construction, and the classifier comes
before the N and is another noun that singularises normally, I'd expect the
classifier to rapidly become an inseparable prefix, so that these nouns are
homogeneised into the rest of the system.  

If /l L/ nouns regularly singularise in a completely different way to
mutating nouns, without funny selectional requirements, that opens the door
to the /l L/-stem paradigm analogically stealing nouns away from the
mutating paradigm.  The /l L/-stem paradigm would be at least as welcoming a
home for further borrowings, at the very least.