Sorry for not answering this sooner, I've been off sick for an entire week.

Right, the short answer is that you cannot create an audit which is equivalent to an onaudit / onshowaudit for a subset of tables. You have to audit an entire instance.

The first thing that I would suggest you do is to log a feature request via IBM Informix Technical Support describing exactly how you would like it to work.

The second thing I can suggest is that you can write triggers to audit certain operations, but you're limited to things that would activate triggers. So you could not, for instance, audit adding an index to a table.

If you expand a little on what you're trying to achieve, I might be able to make more useful comments.