Interesting problem.  You could include raised lettering giving a date of
casting, but that might violate the terms of the trust.
 
Or, you could dissolve a dry pigment such as a yellow or red ochre (a
cheap and common earth color) in a little water and add it to the plaster
before making the cast.  The pigment will alter the drying time, but that
shouldn't be a problem.
 
This will probably work alright if there are no undercuts.  The safest
way would be to make a mold of the original and cast your new plaster in
that.  Remember that you need a release agent (soap works well) to allow
you to separate the two pieces.
 
The more I write, the more I hope you have someone who is an experienced
plaster caster doing this for you....
 
Good luck.
 
Jack C. Thompson
Thompson Conservation Lab
Portland, OR
 
[log in to unmask]