I think you know how strongly I agree with your sentiments here; I have written many times on the subject myself over the years, and I was as dismayed as you to learn that ABBA had re-scheduled the exhibition. Unfortunately, both here in the USA and in the UK, exhibitions are not run by the bonsai cogniscienti (by those who truly understand the motivations and ideals of the art) but by those who truly understand how to sit in a committee meeting and arrive at a series of compromises between their well-meaning but ill-suited colleagues.
As you point out, Marc Noelanders (an extremely accomplished artist) hosts his exhibition in winter, when deciduous trees and conifers are at their best. Not until the artists themselves undertake the organisation of exhibitions without the involvement of the bonsai politicos, and aim them at a wider art-appreciating audience, can the situation be rectified.
I will, however, take issue with two of your points: First, the Ginkgo exhibition (late September) does allow for deciduous trees to be exhibited naked. The first picture in the album of the last exhibition was of one of my former creations, and English elm. Its current owner, Stefano Frisoni, of Italy, had managed to time the tree's induced autumn precisely, and what a fine job he did, too!
Second, in my experience late winter is better than late autumn for pruning. For one thing, there just seems to be less die-back. For another, in late winter you can more easily detect which buds are most energetic, and where tiny adventitious buds are beginning to form. But hey - whatever works for you!