That's true. But software authors and configs do have the possibility to cache some items but not others, or cache them one way and not another. So perhaps I should have been more specific:
Do any of the current caching packages allow selective caching of URL content according to a rule (ie URL matches this domain/mask/regex then cache, otherwise don't)? Or are they all, "all or nothing"?
Do any of the current antivirus/antimalware scanner packages allow scanning either without caching, or using a RAM based (rather than disk based) scanning mode or caching mode, or using a ramdisk for the disk based cache?
That's probably what I should have asked…