It can not be handled by wxSmith, because it contains the proprietary wxToolBarAddon, that can be handled by our xrc-loader (see sdk/xtra_res.cpp), but not by wxSmith.
I don't know, why wxToolBarAddOn was created, maybe some other devs remember (most likely some wxToolBar limitations), but even if we would use wxToolBar directly, it could not be handled by wxSmith, because wxSmith only handles toolbars inside a wxFrame.
Due to this limitations, I created the xrc-file for the toolbar of IncrementalSearch manually (it's not so hard).