Can the repeated microabrasions on facial skin contribute the
thickening of skin somewhat similiar to callus?  It would be futile to
ask that question at the day spas since they are into selling the
services.

Microdermabrasion can cause thickening of the epidermis, according to
studies:

"Histologic changes were observed in the skin after six treatments
when compared to pre-treatment controls. These changes included
thickening of the epidermis and papillary dermis; flattening of the
rete pegs; perivascular inflammation; and deposition of collagen in
the papillary dermis."

http://www.anthonyproducts.com/information/productinfo/microdermabrasioninfo/microdermabrasion3.htm

"Biopsies from all of the [microabrasion-treated] patients showed the
following changes: 1. a normalization of the stratum corneum; 2.
epidermal thickening; and 3. increased collagen deposition in the
papillary dermis."

http://www.skinforlife.com/Article-Histologic_Effects_Aluminum_Oxide.html

I have found no references which would indicate that this epidermal
thickening resembles a callus. Most sites indicate that the thickening
is a good thing; thinner skin wrinkles more easily.

I also looked into this and found one scientific paper describing
"slight fibrotic changes" as a result of treatment.  Fibrotic refers
to less flexible tissue, of the sort you see in scars.  However, a
"slight fibrotic change" might be of no consequence.  On the other
hand, if someone overused this treatment over a long period, then
maybe a succession of fibrotic changes, as well as continuous
thickening of the outer skin layers (which in moderate amounts is a
good thing and reverses the thinning of the skin that occurs with
aging) could give rise to a callus-like reaction. However, there do
not seem to be any reports in the medical literature about this having
been observed so far.