I would be interested in providing tissues to a lab that has experience imaging or otherwise quantifying mitochondrial density in whole muscle, liver, heart, etc.
You don't actually need to use imaging techniques to measure mt density... One of the easiest way is to measure the citrate synthase with a spec. See this publication: http://onlinelibrary.wiley.com/doi/10.1113/jphysiol.2012.230185/full.