FILELOG is meant to be simple, minimal options.
It is not too hard create your own completely customizable log mechanism in its own PROGRAM code block using FILEOPEN, STRPRINT, FILEWRITE, FILECLOSE. Stick whatever you want in your STRPRINT.
I would recommend using Stage
Name the PROGRAM codeblock MyFileLog
SG S0
FILEOPEN (append) OnSuccess JMP S1, OnError JMP S3 // always try to close the file
SG S1
STRPRINT
JMP S2
SG S2
FILEWRITE OnSuccess JMP S3, OnError JMP S3 // always try to close the file
SG S3
FILECLOSE OnSuccess JMP S4, OnError JMP S4
SG S4
EXIT
Then RUN that code block at whatever interval (e.g. SP4 for once a second, your own auto-resetting timer at whatever interval you want T42.Done)
// somewhere in $Main
STR T42.Done
AND C99 // completely disable it by turning off C99
RUN MyFileLog