The plugin filter mechanism is now almost complete. There are still a couple of issues but it is mostly there.
The attached archive contains a built version (debug build on my FC4 box) of the filter test framework. This can be installed by copying XPFilter into /usr/local/bin (or anywhere else on your path) and copying the xpfilter.xml file into /usr/share/xaralx/filters (you will need to create this folder). You will need to be root to do this.
After doing this, the latest XaraLX build will have the Plugin Test filter available for export (which will just save normal .xar files). The filter will also be available for import (though it currently has no HowCompatible function as this interferes with file loading).