Proto-caucasoid as I understand it would mean the population from which the caucasoids evolved. Any evidence that the Ainu actually are direct descendents of that population?
Genetically, the idea of proto-caucasoids are obscure, because the phylogeny of proto-caucasoids would not choose them to be common ancestor of Caucasoids. When I said this term, I confess I don't have much idea except certain skull feature.

I have no proof, either. The idea of grouping ainu/jomon into mongoloid or caucasoid, is rejected many times. I presume that people just made up and use the proto-caucasoid which does not fit in the description of more generic mongoloid, caucasoid, and negroid. Ainu/Jomon is unique and does not need to be classed, but because scientist likes to group them up, they invented a few obscure terms. Still, proto-caucasoids are not the best term for any of modern races, because of admixtures, and environmental variation in the population.

Aborigenes are proto-caucasoid, too? Amerindians, as well? Caucasoid ancestry must be very heterogenous, then.
I need to warn you that I assumed too much regarding Australian Aborigines (they also have racial mixtures within). There is a study of comparing the skulls of Kennewick man, and some residents in Oseania (or Australia) showing some proximity. I said I used this result, but as minty pointed out, this is the weakest assumption. I don't have any support evidence backing my assumption that these two share the closer common ancestor either, I mean genetically. So this idea can be rubbish, but more or less an idea I advocate,

Morphology, hmm? Not very convincing, esp. since Ainu are said to be derived from Jomon. Skulls give hints, but not more, IMO.
The field of linking up genotypes with phenotypes in evolutionary setting is new, and there are no serious studies published yet, accoding to the professor I talked to. Morphological results will be useful as there will be findings on the genotyped markers of these facial features, some day. For a moment, it will be a speculation, and stays in the level of social or anthoropological science.