When used correctly, the high intensity rear is very effective and doesn't lead to someone coming closer from my experience. It's just that there are few times when they really are needed and we are plagued with people that see a wisp of mist and turn the damn things on for the rest of the day.
I figure it's very easy, if I can see someone 100 yards ahead, so can the person behind me see me and there's absolutely no need for the light to be on. On the days when it is pea soup, it helps but even then, when someone has clearly come up behind and is then maintaining distance, I will often turn mine off again.
The problem is when people start turning them on because for a moment, they can be mistaken for brake lights (even with one - cos we've all seen cars with faulty bulbs) and then someone does brake and that can start a braking cascade.
I'd rather they had been a strobe because it's far easier to notice a flashing light than a continuous one, in fact i'd rather brake lights were a very rapid strobe but it's too late to change all those over.
If you want to hack yours around though, wiring would be trivial. Both bulbs have an earth so on what was the rear, just connect it across to the switched wire on the other bulb. You could try to source some frosted translucent material and cut a piece to fit but the thing is, those rear panels are awfully expensive, even on ebay.
David.