It can be done. Shit, anything can be done. It seems that responses to your question are wondering why you would do it, if there is a better option already installed. As mentioned, you could re-route the tender plug wire and put the plug smewhere else, like behind the side cover or even mount it on a little bracket under the saddle bag, so it's not visible but easily accessible.
If you really want to do it through the cigarette lighter then you can dsconnect the lighter from the wiring harness (unplug the hot and ground spade connectors) and then run a new pair of wires directly to the battery. I'd add an in-line fuse just for peace of mind. Then your cigarette lighter will get power all the time. This isn't necessarily a bad thing. I'm planning to add at least one 12v charging point to the bike that is always hot, most likely inside the tour pak. It'd be nice to be able to charge my iPad or GoPro while the bike is parked. Just have to remember to keep it to small items that don't have a lot of draw on the battery.
