Statement warnings do clear for each execution of a statement, however from a client prespective a continuous execution is a single statement execution. Even in the normal query execution case it is possible to accumulate too many warnings on the client, as we don't yet have code to bound the accumulated statement warnings.
In general the style of the continuous execution is entirely up to the query and the sources. If you have a query that doesn't specify any post-processing operations over the entire result (no sort/grouping/etc.) and the translator never indicates an end of results, then only a single iteration of the plan will be executed. More typically the sources will only answer for results over an interval and the plan will be reexecuted on the server side many times. onResultSet only makes sense in the latter case, but can be easily added.
Statement warnings can be cleared with the clearWarnings method. It could also make sense though to add warning handling on the callback rather than accumulating on the statement.
Steven, I ended wiping out the warnings from statement between re-executions using clearWarnings method. I'll probably get back to this later with more thoughts, but for now I am fine, thanks!