While isql does not have a command option to turn the display of headers on and off,
the sqsh client does have such a command ("headers"). So in terms of having this
capability under the control of the end user, I'd suggest using sqsh rather than isql.
It doesn't really help in this instance though (for reasons discussed by others)
as you want to control the display from the procedure running on the server side
- where your code doesn't really have any way of knowing just which client application
is executing the procedure anyway. (Just because the client application claims to be
"isql" doesn't mean it is the isql that SAP provided with ASE...).
-bret