You could try hanging some weights off the neck whilst the guitar is inverted, say on a table with the neck sticking over the side and some weight on the body to keep it in place. Constant force might help pull the glue joint apart slowly. Nothing too drastic, as if there is a well glued piece, it's the wood that will give, not the glue.

You might then try heating the join with a hairdryer, as although the heat may not melt the glue, it might soften it a bit. There are quite a few materials that soften a long time before they actually melt.