I checked that I had 64-bit Domino installed. The osgi properties I had were ws and os both "win32" and arch "x86_64", which both seem to match the restrictions for the 64-bit plugin.
I imported the plugins and features into Eclipse and rebuilt without the restrictions. See screenshots and attachments, in case I've not done it correctly! One thing I noticed is that the build.xml was removed from the plugins by the build process, so I don't know if I'm missing something in Eclipse.
When I tested those plugins, the win64 one threw the same errors. The win32 one threw different errors:
HTTP JVM: java.lang.UnsatisfiedLinkError: C:\Program Files\IBM\Lotus\Domino\data\domino\workspace\.config\org.eclipse.osgi\bundles\18\1\.cp\lib\domsql.dll (C:\Program Files\IBM\Lotus\Domino\data\domino\workspace\.config\org.eclipse.osgi\bundles\18\1\.cp\lib\domsql.
HTTP JVM: ll isnot a valid Win32 application. )
There was also a stack trace starting with java.lang.ClassLoader.loadLibraryWithPath(ClassLoader.java:1011)

