Thankfully it looks like good news on that front as the official Xbox Support Twitter account has responded to such a query confirming that it will be supported.
"@SomberShadowI Hey there, good question! While the console allows it, DLC of specific titles is up to the publisher :) ^GC"
Microsoft said that all Xbox 360 games will be supported by the software emulator it has created for the Xbox One, but that allowing support is still the responsibility of the individual developers. The exact same process seems to apply to DLC as well, so hopefully all the boxes will be ticked and we'll be able to play all of our Xbox 360 collection.
We took a look at backwards compatibility, currently available to Xbox One Preview members, and found it to be an extremely slick process. For more on that check out the link below.
Source: Xbox Support