When iParts contain Workplanes etc. they are not displayed in my assembly, the iPart is inserted to. If you manually include those assets via the iPart table (which in my understanding should more of an opt-out), they get displayed in the window but not in the model tree browser.
Still via the selection priority "Elements" you can select it and press Alt+V to hide but there is no way to select it anyhow to make it visible again.
A dirty workaround I've found is: make the entire part invisible and the visible again - then every asset is set to visible as well...
