On many networks, the method to write in queries (private chats) will not re-state the idle, because are many scripts who use this method: the script sends a specific message to yourself and halts it, in this way the irc server update your idle to 0. The only way is to send a hidden message to a channel, but in this case you must join a channel, lock it and set the script to send there.
Interesting, any networks in particular so I can test on?