By default the configuration option DenyNoIndexField is true which means that XPAT and XHDR searches for headers which are not in the overview.fmt will be disabled. To enable searches for all possible headers fields put this option to false (0).
Allowing searches for arbitary headers mostly means the backend server has to traverse all articles in the spool looking for headers. Known headers can be looked up in the overview database instead of the spool. Traversing the spool can mean a drastic load on the backend server.
In short, NEWNEWS only works for single groups instead of wildmats. This is because of the same reason NNTPSwitch has to be connected to a backend server. Using wildmats could mean multiple server connections. Probably the full version of NEWNEWS will never be supported by NNTPSwitch.