You could maybe simply use a black pixel width line on the right boarder to hilight the edges contour of the border that side to try and define the play area better. That might be enough but I understand what you are saying in needing to make the edge stand out.

With the suggestion blaster made you would not actually need to swap out tiles for the shadow effect. You would simply change the palette number for that tile that needed shadow in the background (so its even easier). In effect you would have 2 palletes for the background. One where there is no shadow (normal) and the same palette again but darker (where there is a tile shadow). This would allow you to have a true shadow effect in the background providing the brick tiles are always alligned correctly and never move. The only pain may be designing the backgrounds taking the shrinking into account. Once you did it though you open yourself up to new graphic effects during the game.

The other thing I feel you need to address are the falling pickups although I'm not sure if what you have now (the arkanoid type) are just placeholder graphics. I never liked those in the original arkanoid game but they do not fit with the other graphics of hypernoid. I feel it would look better with larger falling rotating 3d rectangle bricks (slightly smaller than the bricks you destroy with the ball), or even falling eyes that are inside the bricks. If you also make the shadow for them less width (so they appeared closed to the ground) you could make them drop under the remaining static tiles instead of over them. I'm not sure how that would look but it may look nice. Espicially if they disintegrated if they hit the guard and only fell off the screen when the guard was inactive.

I also like the small effect you added with the guard shadow btw going black when the guard is hit.