Import PCB outline made from connected lines from Inkscape
Since you designed the outline yourself, just re-enter the same information into Eagle.  Eagle lets you define the outline as line segments and arcs.

I just looked at your first link, and it seems the real problem is that you are trying to do something completely unrealistic:



First, if you are designing a PCB, you should do so in the program for that purpose, which in your case is apparently Eagle.  Second, you have to understand how printed circuit boards are made to avoid impossible specifications.

Each board will be cut from a larger panel by computer-controller router.  These have straight lines and arcs as primitives.  This means most of the time arbitrary curves have to be approximated with lots of small line segments, which is inefficient for the board house and very costly for you.  Usually after 6 or so route points, they start charging per route point.  Every line segment is one route point and every arc two.

Another thing you have to think about is that there can never be a true interior corner because the router bit will have some minimum diameter.  If you specify a sharp interior corner, you don't know what you'll get.  The board house might use the smallest router bit they have and charge you extra for having to change bits or for the small size (smaller bits break off more easily and cost them more), and you still won't get a sharp interior corner.  It is best to always specify interior corners yourself as arcs of some reasonable radius.

All in all what you are trying to do is unreasonable.  At the very least, talk to your board house first and find out what special things they might be able to do.  It is also unfair of that web page to say that Eagle has a "glaring omission" in this area when the glaring omission is really in their knowledge of PCB fabrication.

If you really need such a strange board shape and this is a one-off, it is probably better to have the board house cut your board to the minimum enclosing rectangle, then cut the wierd shape yourself.  It might require a combination of using a band saw and some manual filing.  Make sure there is no copper in the areas you don't want and within some minimal distance of the edge.  The copper will dull whatever tool you are using to cut or grind the board.  You want to cut just fiberglass, not fiberglass and copper.