May I call this “defined” 
but is it “specified”? What is needed, is a reliable base for a third-party software.
Maybe this post from Mai about documentation of xmp, which has had the same background meaning, helps for clarification.
I hope we do not stuck here, by discussing, whether current xmp would be suitable or not, whether base64 is usable or not. I wouldn’t have spent the efforts to write this article, if I could have succeeded with such approach at Gunther 
As I said above, once we proceeded in a way, coming closer to “yep, the idea is doable with a separate view and ‘castrated’ modules…” then @gwegner might even support in defining which elements/classes need to be specified for xmp to even have a chance darktable and LRTimelapse can be compatible one day (to prevent, every now and then things suddenly behave different and the interface wouldn’t work anymore which makes users to think bad about both softwares).