I believe that they insist on their integrated clamp because it bears their trade mark. It's a lovely engraved clamp. Here's the site with photos if you're interested.
Riding in Circles | Custom Frames

27.2mm is the standard because the sport evolved from road and track bikes. There are advantages to having your components being compatible with other polo bikes. This way if something fails, say for example your seat-post, then you can borrow another one for the rest of the match.

I think the play is to agree to their clamp and lengthen the ST extension just in case it someday fails.

Thanks for the input guys!