Guess I don't see it either, but I seem to remember Mitel Mail having that as an FCOS bit or maybe I am just losing it a little bit.
BUT... Have you tried adding FCOS bits 48 & 49? It would be partially what you are looking for...
048 No Auto-timestamp of Unplayed Msgs
The server will not play time stamps for previously unplayed messages, and the user will not know when messages were actually received. Use this when the user has a voice pager with limited time. To hear the time stamp, the user presses T while the message plays, or T-1 if the FCOS includes feature bit 204 (Message skip, forward and backward). The user cannot play the time stamp if the FCOS includes feature bit 144 (Skip forward to next message). When a user replays a message, the time stamp plays if feature bit 049 (No auto-time stamp of played msgs) is not in the FCOS. Requires feature bit 050 (Play messages).
049 No Auto-timestamp of Played Msgs
The server will not play time stamps for previously played messages, and the user will not know when messages were actually received. However, if that user gives a message to a mailbox without this feature bit, the time stamp plays for the recipient. To hear the time stamp, the user presses T while the message plays, or T-1 if the FCOS includes feature bit 204 (Message skip, forward and backward). The user cannot play the time stamp if the FCOS includes feature bit 144 (Skip forward to next message). Requires feature bits: 050 (Play messages), 053 (Keep messages)
You could also add FCOS bits 82 & 83 and just let the user skip it.