We've already got the code for customizable buttons working, actually; the only reason we haven't put it in yet is that there aren't any other buttons working besides the ones that are currently shown. We replaced the flashcard buttons in the last version with cut/copy/paste in this one because the "disabled in preview version" error message was confusing a lot of people.
And as far as that copy-and-paste ability, that actually will work if you're using CJKOS/ChOS/etc and enable the "Input field compatibility mode" - it's not perfect, but it does at least convert the tone marks into tone numbers so that the dictionary will search for the word correctly. The reason we haven't yet enabled it with "Input field compatibility" disabled is because we haven't added support for homonym numbers yet and because we're working on revamping the "see also" texts - if we have time we'll probably try to switch them over to using characters instead of pinyin, but at the very least we'd like to make them hyperlinks or something like that. (much of the code to enable this is already in place, it's just a matter of going through the dictionary text and tagging it appropriately)
And nubix, you're very welcome - after all the effort we've put into this, it's very reassuring to hear that even at this stage the imrovements over Oxford E&C are that significant.