Currently, gnuplot does not permit the "set samples" parameter to determine the sampling of splot isolines when hidden3d is implemented. This strikes me as a limitation that could be overcome with some attention. It's quite alarming to apply "set hidden3d" and have a gorgeously rendered splot turn jagged. And it's disappointing to not be able to use the gorgeous higher sampling with the hidden3d option; I hate having to choose when I really want both! I'll be happy to tackle this will a little bit of help.
For example, the jaggedness in these plots (http://www.gnuplot.info/demo/hidden.html) could be alleviated if "set samples 100" affected them like they do when hidden3d is not set.