We have two types of plug-in filter.
a) Import / export filters. We hope very soon (within a week) to have updated the Xara LX filter system to allow easy integration of third party import / export filters.

b) Live Effect filters that use GIMP filters like we use Photoshop ones. This requires that we port the Live Effects tool code first and then that someone replace where we call PSD filters with calls to GIMP ones instead. That would be great and hopefully can be done at some point (Live Effects code porting is a little lower down the priority list for us).

If you (or anyone you know) wants to have a go at this please just post to the dev mail list and we'll try to help you do this work.

We also hope that we can get actual Windows Photoshop plug-ins to work as Live Effects (assuming you're running on x86 of course). This will be a little bit down the road though.