Can the ptype definition be modified to always start an instance (whether or not one is already running)?

No. Messages to a ptype are blocked during start-up until the ptype either replies to the message, or issues a tt_message_accept call. However, the implementation of the ptype can include tt_message_reject for any request it gets that do not have a status of TT_WRN_START_MESSAGE. All requests will then be delivered to (and rejected by) all running instances of the ptype before a new one gets started. This method will be slow if many of these ptypes are running at the same time, or if the message contains a large amount of data. Alternatively, you could use tt_message_accept, which basically unblocks messages to the ptype.