If there are any libraries that have used this text message alternative to the token, could you please share your experience?
I think you may be referring to the WWWOption ICON_FULLRECORD. We are using
it in our catalog (http://roc.rodmanlibrary.com/). The holdingsmsg token
even with additional text doesn't really explain that even though no copies
are available for checkout, that there may be a reference copy or one in
storage. We included the ICON_FULLRECORD text version to lead users to the
full record. It seems to be a good alternative.