Another thought might be to ditch the renderer approach entirely and to create a custom subclass of Accordion along with a custom skin extending org.apache.pivot.wtk.skin.terra.TerraAccordionSkin.
(Or just copy both classes and edit to create your own parallel versions rather than extending)
I haven't prototyped it, but I think it (or something similar) should work, although I'm not sure it is necessarily preferable to just managing the repaint callbacks yourself.
TerraAccordionSkin contains these 2 inner classes which deal with the Accordion headers and will normally use a supplied renderer to render the header.
might override these so that the header is a simply a real BoxPane with
an ActivityIndicator and a Label inside. Then the BoxPane skin (and
ActivityIndicator & Label skins) will take care of painting, and
more importantly, animating.
(ie, use real, fully fledged Components, rather than a Renderer for each header)