Seat Collar Clamp with Rack Eyelets
 

I am using one of these to mount my rear rack, due to a lack of braze-ons:

https://images-na.ssl-images-amazon....KL._SX466_.jpg

The frame is steel.

Will this in any way structurally compromise the bike frame, or create any other problems?

Very unlikely. Keep in mind that most of what this attachment does is keep the rack upright. It will take some of weight, but most of the weight will be on lower rack mounts. Although if your bike lacks upper rack mounts, does it have lower rack mounts?

On my wife's bike, I threaded the rack supports right through the quick-release of the seatpost clamp. Always meant to get one of those, but never got around to it, but she never had a problem. She never did any loaded touring, either.

I'll echo what Rob_E says. I'd add is that you'll probably need longer rack stays. You may also have a problem with the angle on the rack stays on larger frames. This isn't that much of a problem for flat stays but round ones might be difficult to mount since they mount under the rack.

For small bikes...like 12" frames...using a seatpost clamp mount might result in shorter stays. This bike for example, would probably work better with a clamp mount

http://i144.photobucket.com/albums/r...e/DSCN0576.jpg

We've used one quite a bit on my wife's 50 cm compact frame road bike. I made the panniers with a lot of taper to reduce heel strike. It should not cause any damage. The issue we had was the small frame.

The red bike in the front is one of our daughters riding it. Can you guess why we called this the Teal Trio Tour?
Both daughters are riding LHTs now:)
http://i783.photobucket.com/albums/y...892%20copy.jpg

Not a good picture due to the amount of enlargement, but it gives you an idea of it how it looks.
http://i783.photobucket.com/albums/y...oad%20bike.jpg

Rack unloaded.
http://i783.photobucket.com/albums/y...ab096f912e.jpg

No.

In fact there's a major advantage to this approach. If there's some kind of failure, ie. stripped threads or snapped ears, you're only out a cheap, easily replaced part, vs. having to seek a frame repair.

Thanks, everyone.

I assume you do not have a carbon seatpost. If not, that clamp should be just fine.

Might drill it, and use an Easy Out

I do.

Why would that matter? (This doesn't go on the post, but the steel frame, replacing a similar one without the eyelets that was there previously).

I thought of trying something like that, but I really don't need it, and it is an expensive custom frame.

If the clamp goes on the steel frame, then it probably is ok. I have not used any carbon stuff, I was making my comment based on what I have heard about not putting anything on a carbon post other than the saddle. If in doubt, ask the manufacturer or bike shop.