The wiki says Warlight doesn't support lines except for solid, and trying to upload dashed lines they do convert.

Some maps though do have dashed lines showing sea routes. Is this something that just used to be possible, or is there a trick to making it work?

https://www.warlight.net/SinglePlayer?PreviewMap=16114

https://www.warlight.net/SinglePlayer?PreviewMap=24843

I hand drew the dashed lines I preferred, grouped them and just object>path'ed it

Draw the line and choose the dashed pattern you prefer. Then you click the line and press ctrl+alt+c or path - Stroke to Path and there you go, a dashed line.

For some of my maps where there are dashed sea routes I simple created a line the width I wanted, copied it and changed curvature to make the sea route look even better.

Thanks for the help, easier than I was expecting. :)