Hi Dave,
The ability to browse large trees seems to be a particular limitation of existing tools (I'd love to be corrected if I am wrong). Having a tree larger than the widget, as in Phylowidget, is one approach, however, an overview window would be nice to orientate your view in relation to the entire tree. I have also been considering displaying only a subset of nodes and then having 'expand', 'contract' and 'pan' (by expanding and contracting) functions for navagation. The ability to display node subsets is probably more important for networks than trees as reticulation will often result in visual occlusion.
Rod Page has coded a web widget (I believe all javascript) that has a small preview window for the whole tree and a larger "zoomed in" view. "TreeJuxtaposer" is a java app(let?) that allows you to contract and expand selections of nodes/clades. I think these come closest to what you are talking about, though neither operates on networks.
Cheers,
Rutger