Malcolm,
If the desired functionality is to group a bunch of documents, (and not a specific number of documents), you could keep the trigger in a suspended state and use a scheduled IS service that would reactivate the trigger every <time> minutes (services in pub.trigger can suspend/enable triggers). The handling service can then retrieve documents and append to an IDoc. The tricky part would be to share the in-process IDoc among successive invocations of the handling service, and to signal end of queue to publish the IDoc to SAP.
Alternatively, you could use a scheduled java service that uses the Broker API to fetch documents from the queue (you could eliminate the trigger entirely and just use a broker subscriber client). Here, you can fetch as many documents as you want in each run, hold the IDoc in memory in the same service, and know exactly when to send the IDoc.
Hope that helps,
Prashanth
#Integration-Server-and-ESB#broker#webMethods#Universal-Messaging-Broker