I was checking Alice and her bubs tonight before everyone got tucked in and covered up for another night of very cool minimums... when the 'pinkness' of the bubs caught my eye. I scooped them out, and found that they were pretty muchly bald!
Constance (above) is 90% bald. Just a bit of hair on her head and feet left. Cuba (below) is maybe 60% bald. He looks particularly bad! They bubs are 3 days old today, so it's not long enough for something like mites/fungal/lice etc to have caused such a dramatic hair loss. And mum is fine - she has a gorgeous thick, full coat.
So my next thought was, maybe mum is over-grooming them. So I've bought the whole family right inside with me so I can keep a very close eye on them. The other options are some kind of genetic quirk (like the baldwin breed of guinea pig who are born furred, but loose their coats to become completely bald), or a hormonal imbalance/dietary deficiency.
At this point it's going to simply be a matter of keeping them warm, well fed and waiting.