I'm not against adding a basic entry if it's not already in, but I have the feeling Doris would probably draw the line at a long description and say LEO isn't an encyclopedia. Something really short in italics, like '(association of Lutheran, Reformed, and United churches)' or just '(Lutheran, Reformed, United)' might be more feasible.
I don't think 'Protestant' is unduly misleading in certain contexts, because the EKD is the primary Protestant church in Germany, and similar in some ways to a state church, unlike the much smaller denominations outside it.
But the place I was really talking about an explanation in parentheses is in each text to be translated. Different contexts need different explanations; in a passing mention, Protestant (as opposed to Catholic) is often enough, but in a more involved discussion, you might want to mention the 3 constituent denominations, or maybe even the 22 regional church bodies.
So as much as I think an explanation often helps in translation, I'm not sure it makes a lot of sense to add any lengthy explanation to LEO, when the EKD does have its own website, and LEO does have quite a few of these archived threads (several of which got dug up today, for reasons not entirely clear).