Sounds like you're referring to this issue
where we discussed allowing filter authors to apply arbitrary styles.
Sounds like a similar issue, anyway.
It wouldn't have to allow for arbitrary styles, though. Understandably that could get a little hairy! You could implement as only a limited selection of blocking behaviors to choose from. E.g. normal blocking (don't fetch the resource), block visibility (hidden), block display (none). The latter two both fetch the resource, of course, but do or do not affect the layout, respectively. Both may be useful in differing situations.
You're right that either of these could be as easily detected as complete absence of the ad, but I don't know what you could do that isn't detectable somehow.