As a alternate search engine, there is http://www.cranialinsertion.com/cardsearch/
There is an option under colour to use colour identity but this is more geared towards "If I have xyz coloured commander, what cards can I use". With a bit of creative searching, I have made the following queries:
(Gives you all the legendary creatures you can't put in all four colour or colourless commander decks)
(Gives you all the legendary creatures you can use in the particular four colour combo but not can't use in the other four colour combinations plus colourless commander decks)
There is also the legendary creatures with partner but you can quickly find them by searching for partner in the card text (Commanders with partner)
As with gatherer, when you conduct a search with only one result, it will redirect to that card page.