It's about ten years since I was in Luang Prabang. At that time there was one gay bar and a mixed disco. I had booked an evening tour with the gay travel site Purple Dragon. It was not the most exciting evening but the guide was gay and very cute. I realise you are making your own arrangements but if you wish to contact Purple Dragon for advice, their site is - http://www.purpledrag.com/laos/index.htm