Screenshots in hol are not actually restricted to 320x256 - you can put larger screenshots in which are scaled for the thumbnail view to 320x256, but if you view the image popup you get the full size. Only exception is the DBS, and since the purpose of this is to give a quick summary image of the program I don't think it is really a problem (no different to games that run in hi-res or interlaced modes).
Quite a number of hol fields are a bit irrelevant for apps, but I think we could get apps in to current hol template. However there are certain issues that would need to be worked through eg how to treat different versions. Also we would want to keep the apps separated from games.