Ok, added an init-start script in /jffs/scripts like this:
#!/bin/sh
echo 4194304 > /proc/sys/kernel/pid_max
logger -t init-start "Set pid_max to $(cat /proc/sys/kernel/pid_max)"
Rebooted.
pid_max got updated, the log line did not appear in the syslog.
I'll watch what happens now.