It'd be a simple enough browser plugin to build - a tool that checks multiple trust stores when rendering a page. Probably it already exists.
The problem is between the keyboard and the chair. Users struggle to understand SSL already. Browsers decided that the distinctions between EV, DV, and OV were too complex and hid them. What will your grandmother think when she opens up her bank and your browser plugin shows a greenish yellow trust indicator because the cert is trusted by Google, Apple, and Microsoft, but not Mozilla?
Unfortunately, trust is binary. Your grandmother click on the bank bookmark and either sees her banking websites or sees a scary warning.