browserstack reports with protractor
From what I can recall, there is no mature reporter that can aggregate results from different capabilities. There is that protractor-multicapabilities-htmlreporter_v2 that is close to what you are asking about, but it's not active at the moment and is not widely used:
You can also dump test results with JUnitXmlReporter (separate report for each browser/capability) and then postprocess the results and create a combined report. There is junit-xml-parser library that would help you to parse the JUnit XML reports. You would just need some custom logic for aggregation part and creating the resulting report.
But, in general, I really like the idea and also need a similar functionality.