UNADVISABLE. Biggest makers of leather stamps use soft, easily
machined brass for stamp construction, and chrome plate the
stamps. The plating protects the leather from incursion of brass
particles that would patinate (turn green) with age and leather
treatments. Alphabet and large-area stamps are (I believe) a cast
bronze, white metal plated. Way too soft for metal. Do not rule
out as applicators of an etching ground.
Steel stamps present a particulate hazard and would rust in the
leather, spoiling the stamped design.
Conceivably, you could stamp leather with steel tools, HIGHLY
POLISHED AND VEGETABLE-OILED!!!