There is no support for adding features to ideograms in Gviz. You could of course combine the ideogram with another track type, but I guess that is not what you are looking for here. There is also the OverlayTrack feature in Gviz,which allows to superimpose multiple tracks, but since IdeogramTracks are the only tracks in Gviz that somehow break the contract of having identical horizontal scales, a simple overlay will also not be that trivial.
Of course you can always just take the IdeogramTrack object, plot it using its drawGD method in a viewport that you fully control and add whatever you please on top of it.
Hope that helps at least a bit,