You could make an SSRS subscription that writes the report to a windows share then display those files in a browser. The file formats are a bit limited but I believe it supports html and tiff. Along with a simple bit of html/javascript to make the browser reload the page every X minutes and you could have an ok dashboard.
Of course this means you'll need to use the SSRS reports rather than the newer tfs charts, but you can create your own reports using the warehouse.
TFS online supports basic auth for alternate credentials but it appears you can't use this on prem (yet). Also in the future it looks like you may be able to use the built in charting tools (they can be embedded into team dashboards now), so potentially with 2013 update 2 you'll be able to make a better dashboard.