I've entertained the idea of Celis bars on the rear, but not sure how well they'd match the E53.

The bars themselves aren't anything more than a light pipe let at one (or both) end(s) that operates in a similar fashion as a prism. The difficulty isn't in getting them out of the e83 tails, but properly into the e53's. If you could get a set in the cheap it would definitely be worth a shot IMO!

I'm accepting parts donation to give it a try. Any taker?

It might involve moving the reverse light or rear fog lights. Depending wether you want the bars right below the top hatch, or at the bottom of the lights.

I would want a set of clear, white tail lights to use the glass part from. That way you can move the reverse light, mask off where you moved it to on the glass, and paint the rest with red transparent paint.

The bars themselves can be replicated, but the intensity of the light and the OEM-look would be harder to achieve. That's why the E83 bars seem like a good solution to me.
The inner part of the E83 tail lights are shorter than on the E53, and the outer part of the E83 lights may have a different curve than the E53 ones. If they're made of plastic that softens with heat (some types of plastic will stay hard and just crack) they could be heated and bent to the optimal curve for the E53 lights.

The outer bars might also be too long for using them on top of the hatch, so mounting them at the bottom would maybe be the easiest.

These are just ideas. I haven't measured X3 tail lights, I have just compared it in pictures.

Opening up the E53 tail lights, might be as simple as just carefully sawing the edge and carefully breaking off the glass/lens. They look like they're just melted together on the edges.

A year or two ago, I modified a pair of E34 Touring taillights. For making 1 set, I needed two pairs of taillights. They're melted together practically everywhere. On set number 1 I had to break the lenses, to have a clean rear part. On set number 2, I had to saw and break the rear part in pieces to be able to get the lenses off without breaking them.

I guess I won't be trying to modify my E53 tail lights, as I'm considering selling my car and getting a nice E32 or something. In Norway, the facelift E53 costs from $40 000 and upwards. The used parts prices are also high, because the cars are still expensive... I guess I'll be buying a E53 again some time, when the cars and the parts are more affordable. Then I might try modifying the taillights, if it isn't already done by then, and a DIY is available :)

I just bought a set of aftermarket LED tails that did the trick for updating my rear end, only cost $200, and didn't have to hack up anything or do custom, worked for me.

Just a thought (not a condemnation or put down)...is there any point in time when something "new" is too much...and if you have something older...its best left as is?

For example, 100 pounds & 25 years ago...a 230 lbs 50 year old man could probably pull off wearing a pair of skinny jeans...but at this point and time in his life...it might not looks as good...even though the skinny jeans are more "hip"... a regular pair of "full cut" Wrangler's or Levi's would probably be better. :D

Again, just a thought as the proliferation of LED headlights & tail lights abound...and I become less impressed with them.