With slony 2.2.1 and using the slonik_execute_script, it generates
what appears to now be obsolete set id parameter:

cluster name mycluster;
node 1 admin conninfo='host=node1 dbname=mydb user=slony port=5432';
node 2 admin conninfo='host=node2 dbname=mydb user=slony port=5432';
execute script (
  set id=1,
  filename = '/tmp/alter.sql',
  event node = 1
);

By removing the set id parameter the you can pipe the script to slonik
and it executes OK.

-Dave