You Can't Be Serious!
You've got much bigger problems to worry about if covering less than 10% of your screen completely breaks the game.
The only thing wrong with the screenshot I linked to is the eye being off center one tile to the right which makes the output panel appear to cover more than it actually does.
Moving the eye to the right is what you'd want because it'd shift the player's mob to the left.
You can change how much the eye gets shifted to keep the player's mob centered in the view, even when things block parts of the screen (like when you open your inventory in Diablo). A message log seems like the kind of thing you'd always want there, so I'd just make buttons to toggle its size.
Having partially transparent interface elements (like this thread) seems like one of those things that people think they need (because other games have it), but having it wouldn't actually improve their game. If you have a terrible game, having a message log obscure part of the screen is just another problem.
The simple fact is that DirectX's rendering doesn't play nicely with transparent and translucent windows. That might have improved in newer operating systems so I think it's something we could put to the test and perhaps detect, but it might depend on the GPU as well. Before this was put in, games that mixed transparency/translucency with hardware rendering experienced all kinds of artifacts.
Another potential option we may have is to use an internal DirectX buffer but somehow blit it via GDI in a way that won't suck up significant processor resources. I'm dubious whether that's possible but it's something we could look at.
We've been talking about other rendering improvements that could be done like built-in text without having to rely on DmiFontsPlus, so that might even end up being the simpler option.