Gnome has bonobo
http://en.wikipedia.org/wiki/Bonobo_%28component_model%29
KDE has KPart
http://en.wikipedia.org/wiki/KPart
Both also have a more modern COM/DCOM like API called D-Bus, but as far as i know it does not have a Custom Control layer.
I would suggest using D-Bus as it is cross-desktop and more modern then Bonobo and KPart.
Perhaps a ActiveX-like model can be used on top of D-Bus for custom control creation. As i dont know of one we have the freedom to shape it how we want it to be. For example we could make it similar to ActiveX making code easier to port.
Also i would prefer if no platform-dependent thing is implemented in Jabaco. That would make D-Bus the best choice as it is ported to many platforms including Windows. Using eiter KPart or Bonobo will probably ailienate part of the gnome or KDE community, so neither of them is a good choice.
Of course, for anything non-graphical D-Bus is probably the best.