As I wrote before, finally I manage to correctly handle the stroke and fill in wpg2odg. The result can be witnessed in the following screenshot:
The two landscape pictures opened inside OpenOffice.org are in OpenDocument Graphics (ODG) format, converted from the corresponding WPGs (which are shown in Corel Presentation on the left side).
There are still bugs on some corner cases. When I clean this up, then I'll commit everything so that it's ready for public testing.