I've had the need for this few times already (e.g. Aria engine does that with the page checksum).
It's a table attribute where the default value is not fixed, but set via a server variable. Like in
for tables t1 and t2 the value of opt_val will be 17.
Note that the value of opt_val is remembered when a table is created, later changes of the session variable don't affect the table.