locks and variable substitution

Is it possible to do variable substitution with explicit locks in plpgsql?
Something along the lines of

...

declare
big_string alias for $1;
excl_table text;

begin
--assuming big_string starts with something like table=table1!
excl_table := substr(big_string,7,(position(''!'' in big_string)-7));

lock table excl_table in exclusive mode;

...

Thanks very much.
g.