Things like this are no longer possible as an extension, which is why HTTPS Everywhere lost a bunch of the security features it used to add.
An extension would also run a whole new instance of the browser, which is totally overkill for such a feature.
As for usefulness I remember people saying similar things about showing any form of HTTPS or cert info at all, due to worry that users won't understand.
Users lack understanding because they are kept in the dark all the time. This is the big difference between the old Amiga world and the Windows world.
On the Amiga OS you get lots of visual feedback and so learn passively what the normal operation to expect is.
When the feedback differs from normal or shows something unusual, you do not need to be an expert to realise something is amiss and needs to be checked.
Lets say you land on a site and see the message in the browser that the encryption has been downgraded. What does this mean ?
Downgraded from what to what, and where does the user see that this is normal or abnormal behaviour for the site in question ?
Unless a user tests the site they do not know what cyphers should normally be in use on a site they frequent.
Just having a small amount of visual feedback somewhere obvious is enough to teach people to pay attention.
It can help all of us spot if something has been compromised in the browser or site.
Ok, I hadn't understood the post was about adding a website as a search engine... I can confirm the behavior you describe...
When I search directly on the website, the string shows up URL encoded in the address bar. If I search using the search engine shortcut, the string shows up exactly in the address bar - apparently the website is not prepared for that (nor should it be - I agree all non-ASCII characters should be URL encoded, shouldn't even be an option).