They maintain their own fork and they say that they contribute stuff upstream (not sure how much, though).

Last time I checked (a couple years ago), it seemed that I could use OpenWRT but I would lose some functionality (was it the FTT maybe?).

The Turis Omnia runs a derivative of OpenWrt. They provide a custom UI with an option to switch to LuCI. It's easy to flash it to upstream OpenWRT, which is what I do.