Emily,

I have exhibited in about fifty outdoor sculpture exhibitions.  I have
seen calls for entry for a few shows with standardized bases, but it is
uncommon.  I understand your concern with old slabs being pock-marked
with holes and cut-off bolts.  I have hesitations though about creating
standardized bases and requiring sculptors to adapt their sculptures to
the bases.  To me, it complicates the process and may limit artistic
flexibility.

A few thoughts about using standardized bases:
--If sculptors are required to make adaptor plates, each plate will
probably look a little different, drawing attention to the adaptor
plates.
--Sculptures can vary widely in the sizes and shapes of their
footprints.  You may find yourself imposing unnecessary limitations on
the eligibility of sculptures OR you may end up with an expensive and
complicated attachment system.
--Julia said that "it's no big deal" for artists to make an adaptor.  I
would modify that statement to say: it's acceptable if the show pays
well.  Making an adaptor could easily be a day's work (plus expenses),
depending on requirements.
--Structurally, an adaptor adds an extra link in the chain of potential
problems/failures.

Installing on a concrete pad is convenient, flexible, and relatively
quick.  The sculptor can determine the best angle for the sculpture
on-site, rather than pre-planning using photos.  With appropriate
lifting options, hammer drill, and planning, a sculpture can often be
installed in a half an hour.

So what about those pesky holes and cut-off bolts?  Consider requiring
sculptors to use tapcon bolts, rather than wedge anchors, epoxied bolts,
etc.  Tapcon bolts can be unscrewed at the end of the exhibition and the
holes can "easily" be filled with a small amount of concrete. The
patches will blend with the surface reasonably well with proper
troweling.  If necessary over time, it is possible to refresh the
surface of the slab with a wash coat of portland cement + acrylic
binder, or possibly paint(?).  (Your maintenance department may have
ideas about this.)

I appreciate the efforts of everyone who is organizing temporary outdoor
sculpture exhibitions.  They provide good opportunities for artists and
they enliven communities.  Thanks!

Harry McDaniel