I did actually try it but you are absolutely right: My test went as far as diagonal lines (which still works well by the way) but I would not know how to do curved lines without having to fall back to straight line dashes (which for most cases would not look acceptable). Thanks for the quick reply!
I just started looking into this, so I am by no means a libgd expert. Isn't gdImageSetStyle (or possibly gdImageSetBrush) enough to implement dashed lines. From what I have read gdImageSetStyle is the preferred option to draw dashed lines (since gdImageDashedLine was deprecated although it still seems to be working)