I think perhapse a 'custom log format' option would be better than just an append. This could introduce an accompanying identifier to refer to the logline in its un customised format. for example
Message to chan: (05:18.54) (Angreal): Hey!@
Custom log line setting: $adate $logline
Messege logged: 10/01/2005 (05:18.54) (Angreal): Hey!@
Custom log line setting: $adate $asctime(hhnn) $logline.nots
Messege logged: 10/01/2005 0518 (Angreal): Hey!@
Custom log line setting: $adate $timestamp $logline.nots
Messege logged: 10/01/2005 (05:18.54) (Angreal): Hey!@
The point of the .nots (NO TimeStamp) property is to represent the standard logline (as it would usually be logged and seen in windows) without the timestamp. I think something along these lines would be much better for general use by others than just an append to. It should allow the use of global identifiers such as $atime $asctime $timestamp and so on. Obviously while using a custom log format it would be wise to have the timestamp logs option turned off.
Just a quick thought, im sure somebody would come up with a better solution than $logline[.nots] for refering to the line that would usually be logged, it was just used to explain the idea.