Hi Mayank
I think appending custom information to access log maynot be possible other than what is suggested in the other post, http://forums.adobe.com/message/3005886#3005886
If you want to display some custom information whenever an event like play occurs, apart from the information that access log provides then I would suggest you to check out the authorization plug-in feature of FMS.
You could refer http://help.adobe.com/en_US/FlashMediaServer/3.5_Plugin/WS5b3ccc516d4fbf351e63e3d11a0d662434-7fe8.html#WS5b3ccc516d4fbf351e63e3d11a0d662434-7fe2
The authorization plugin allows you to monitor server events like connect, play, stop etc and access various fields available for those events. It also allows you to log custom information when a particular event occurs by suitably writing the code for it in the auth plug-in. One can write all the relevant events, fields and custom information to the authMessage.00.log which is created alongside access.00.log.
Regards
Mamata