Here's a recent item that turned up on Ebay -- a nice wooden rosary with acorn-shaped marker beads. It's Italian-made, but it's a souvenir from Our Lady of Fatima Shrine in New York state.
The seller speculates that perhaps the beads are made of oak wood, which would explain why the Our Father beads (gauds) might be in the form of acorns. This is certainly possible, although (with the exception of olive wood beads from Palestine) most wooden rosary beads in modern times don't seem to be made from anything special -- often just some nameless inexpensive wood, sometimes ebony or rosewood.
The acorns could also be a reference to the original vision of Fatima, where -- if I'm remembering correctly -- children saw the Virgin Mary in the top of a small oak tree.
However none of this explains the presence of acorn-shaped beads in rosaries made before 1917, the date of the visions at Fatima. In particular, it doesn't explain Balthasar's acorns, a portrait from the 1500s showing a rosary with just such beads.
I'm still looking for clues.
In the meantime, it would also be nice to find some modern wooden acorn-shaped beads to make a reproduction of Balthasar's beads, but I don't seem to find those either. They don't look in the painting as though they are particularly finely carved, and the one modern set of beads I've seen that resembled them were also rather crudely carved (I assume they are "folk art"). Unfortunately I'm no woodcarver or I'd try making some.
Perhaps my erratic "finder's luck" will kick in. I began looking for flat disk-shaped beads several years ago in order to make reproductions of a couple of intriguing historical rosaries that use disk-shaped counters rather than round beads. I looked in vain for about three years: then suddenly "doughnut" shaped flat round beads became popular, and are now quite common in bead stores and catalogs -- semiprecious stone, wood, bone, glass and just about any other material. Maybe two or three years from now, acorns will suddenly become popular -- probably just as I've learned how to carve wood. :)