It seems the only way to accomplish this is fiddling with the registry. This .reg-file worked for my own search plugin:
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\SearchScopes\{5AB8F1FE-2CAD-45FD-8B50-9E5610E4D2B6}]
"DisplayName"="Mysite search"
"URL"="http://mysite/index.jsp?q={searchTerms}&s=1"
"ShowSearchSuggestions"=dword:00000001
"SuggestionsURL"="http://mysite/searchsuggestions.jsp?b=i&q={searchTerms}"
"FaviconPath"="C:\\somePath\\search_{5AB8F1FE-2CAD-45FD-8B50-9E5610E4D2B6}.ico"
It seems the Icon needs to be stored in a local path. I was not able to get the FaviconURL
to work. It doesn't need to have the guid in the filename, any name will work.
Once you have your plugin registered, you can set it as default via IE. If you want to do it programatically, you need to add the following to your .reg-file:
[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\SearchScopes]
"DefaultScope"="{5AB8F1FE-2CAD-45FD-8B50-9E5610E4D2B6}"