Collectible Card Game Headquarters

I would like to improve the experience of users playing without the full compliment of art.

are there legal issues with using the same style of showing the art that this page does: http://ct-magefree.rhcloud.com/cards

basically a barebones card frame with colour. is there a legal reason that this hasn't been included already?

Sounds like a very good idea. Go for it.

The user can download the card frames with the other symbols, so it shouldn't cause additional legal problems.
I created such a downloader and commited it to the repository.
https://github.com/magefree/mage/commit/72408a7f5206c2f9dff0077fa5f53b9a3b69b3e5