I have found exactly what I was interested in.
Code:
public client_infochanged(id)
{
new option[32];
get_user_info(id, "plugin_param ", option, sizeof(option));
client_print(id, print_chat, "plugin_param new value: %s", option);
}
Then type setinfo plugin_param 1 in the console. The value is restored after client restart which is awesome. And there is even an event listener to catch all changes.