several styles available
Firstly, it's not a problem to have the cable housing loop around the seatpost cluster on its way to a cable hanger, just don't make the bend in the housing too sharp and it'll be fine. That's how it's done.
The kind pictured that goes in the center of the clamp requires a clamp with a wide enough gap in the center to accommodate it. Not all clamps have this gap. I have the style pictured on my Obed. It's not my favorite because the hanger is free to pivot and the tension of the cable housing tries to rotate it upward from the preferred position of pointing directly at the brake's straddle hanger. So if you run your brake springs on the light side as I do, this movement takes up some of the slack in the brake cable that would otherwise be available for the pads to open wider on the rim. For cross you'd probably want strong snappy spring tension so this might be a non-issue. Some of the ones of the style pictured have a little foot that braces them against the backside of the seat tube, right at the slot. Others don't.
Another style of hanger is a V shape that attaches at both sides of the clamp, with the apex of the V, where the cable joins, pointing down to the straddle hanger. Wth this style, the seat bolt holds the hanger so it doesn't move. But to use this style you need a bolt that protrudes out of the seatpost clamp at both ends, like the Control Tech style bolt. A clamp that recesses the bolt head into the clamp won't work. I personally think this V style looks the most sano.
The third style attaches like the above V style, between the bolt head and the clamp, but only at one end, is steel so it doesn't flex, and kinda curves inward so it centers the housing and points it right at the straddle cable. Sometimes this style comes with a quick release lever(!). It was originally used on old road bikes that came with centerpull brakes, thus the quick release.
When I find old discarded bikes in the trash, I always mine them for little parts like hangers, clampon cable guides, etc. even if the rest of the bike is useless to me. Surprising how often a 40-year-old road part fills a need. Just built up a towner scorcher fixed gear with a Mafac Racer centerpull front brake that I found on an ancient frame in the trash bin outside the LBS. They kinda laughed at me when I borrowed a 10mm wrench to remove the brakes.
www.thirdhand.com, or is it
www.loosescrews.com, same place, forget which name is current, used to stock every variety mentioned above. then they pared down their inventory, but still worth checking. or like the guy said, if your LBS has been around for a few decades, there's likely a dusty drawer full of this kinda stuff in there somewhere.
