I've created a module that answers PrivMSGs while no users are connected, to let the person writing the message know their messages won't be ignored, but the user is offline right now.
That may look like this:
<MistaGee> You are talking to ZNC and my master is offline. Please leave a message, I will deliver it as soon as possible.
<cYrano> I wanted to talk to you about some example thingy
<cYrano> I have an example problem on my example server
<cYrano> Could you please have an example look, thanks
The message that is sent by the module can be configured by sending a /msg to *autoreply. Users sending messages won't receive the message for every line they post, the message will only be re-sent if the user doesn't send a message in five Minutes.
Both the default message and the time interval can be configured simply by editing a #define in the source code.