It is a bit irresponsible of me to rely on an old memory, but I believe
that a tack piano is a regular piano used in conjunction with tacks,
attached either to the hammers or to the strings, to give a plucked
character to at least some notes. It is, or so I believe, related to
Henry Cowell's prepared piano.
My previous answer, based on a not infallible memory was essentially
correct - the tacks were in fact attached to the felts on the hammers.
A quick search of the Alta Vista search engine confirmed this.
Professor Bernard Chasan
Physics Department, Boston University