You could just have new content as a separate project altogether than just link to it via HTTP.
Then it would require you to re-load an entire project... and they wouldn't be able to go back and forth easily... just not very efficient I'd assume.
Would it even be useful to load layout XML files from Construct 2? Then everyone who wants to design levels for your game will (probably) need a C2 license, and you have to share a significant part of the source project in a ready-to-use form...
More money for you guys, no? In the end, if you're only giving it to people you want to create levels for you.... say if you have a team of level builders for example, then sharing the code shouldn't be a problem anyways... if you even had to. The main reason is for downloadable content, not having to re-download the entire game... just level data. At that point, you already own the source project, and shouldn't be a problem... not to mention, it would be a lot easier for people to implement, than an entire level editor\loader parser. Point to a URL\Filename of the XML data, load it - and treat it like a loaded Layout.