You could install the XPDF? That would give you X10 in-line control of the lights, however I believe you wanted to retain local control so I don't know that the XPDF would be the perfect solution. It would definitely eliminate the weather element since it could be installed anywhere along the wire run, even inside the house as long as you left the outdoor control on.
The more I think about it, if you you really want to maintain local control I would find a weather proof box and install a wall switch like the 2384
http://www.smarthome.com/2384i2.html that does not require a neutral wire to be present. I just checked smarthome.com and see that they do not have the 2384 in stock. I have used this switch and is a great switch and definitely would recommend to anyone who needs to do a quick install without a neutral wire present. You may be able to locate one on eBay or another home automation website.