[Users] session termination

Hi, I've been playing around with TM callbacks and the TM API trying to build a module which mantains dialog info in order to permit session termination. For now I managed (in a very experimental way) to have a function exposed to the script which initiates (appropriately built with dialog info) BYE transactions to UAs and effectively terminates the call. Later this could be done via FIFO or unix socket command taking some kind of "dialog identifier" parameter... But sending BYEs to terminate calls only works if the dialog is already in the CONFIRMED state. For early dialogs, CANCELs should be used. But in the TM API there is no cancel function. Is there any other way of canceling a request on behalf of an UA from within an (open)ser module? Or would the TM API need to be extended? Thanks in advance for any insights. JF