You can add cron items directly to the config.xml with the latest snapshots. To see the syntax just open a config.xml of a recent snapshot and have a look at the very bottom.
Keeping logs at embedded systems at least is not possible as we don't write them to disk due to limited writecycles of cf-media. If you really want to have historical data use a remote syslog server. At least in non home environments (and even in those sometimes) there should be a machine around that can capture the systemlogs as this is not a heavy task. I even have a server running at home that has some tasks setup to produce today, yesterday, current week, last week rotating logs.
Btw, newer snapshots have a states rrd graph. It should log if you reach the limit. Maybe view that graph to see if this really is the case when the problems occur. You even can view it after reboot (if you are not running the embedded version or livecd as you have to do a clean reboot from gui or shell to write the rrds to disk).
The only solution is to configure squid as acceleration proxy and set up a ssl-cert manually. Then squid is able to interrupt ssl connections and handle them by itself. This should ensure logging https connections without having to set anything up withing the client os.
But, unfortunately it seems that the squid package was compiled without the appropriate options.
This is the complete message? There is nothing in front of it telling which service caused the log entry? Also what have been happening before you get that message?