I just looked into the syncevolution code. It has a gtk based UI and the
backends.
I think the easiest way would be to write a tde backend and use the gtk UI
by opensync. This also means that the functionality will go into the
syncevolution project. Originally I was thinking one could integrate the
syncevolution into the existing tde, but from what I see it would be much
easier to provide an option for sync with tde from syncevolution.

Do you know if it is possible to write KPart for gtk application?

Thanks

regards