All messages sent to the activity (online) log are going through the logproc ($LOPROC).
As it is created as an OML procedure, it is not possible to alter for custom settings. A logproc exit is provided for this purpose and may be used for messages monitoring from the activity log. It can be set through the 'Log Exit Name' field in /PARM $NM LOGFILES group.
Why does message not sent to the activity log with &WRITE LOG=YES?
&WRITE is used to generate a message from a NCL procedure. With LOG=YES parm, it is possible to send the message to the log from any environment except from the LOGP.
For this reason, &WRITE LOG=YES will not send any message if it is executed from a logproc exit running in LOGP.
The workaround consists of submitting another NCL procedure to a background environment from there.
Instead of &WRITE LOG=YES in the logproc exit, use the following statement:
&INTCMD SUB BSYS -EXEC ncl_proc
ncl_proc includes &WRITE LOG=YES
The logproc exit should be used with care in any environment receiving a lot of messages in the log.
Each of them causes the logproc exit to be executed and may impact the performance of the region.