The Hacker News codebase is in Arc, and will be for the foreseeable future. All the core domain work is done in it, and we write Arc code every day. When it makes sense for performance or practical reasons we call into Racket, but not frequently enough that we've even bothered exposing nice syntax for it (like anarki's $).
Good error reporting is pretty disagreeable to me, lol. :-p
I think Arc's main advantages over Scheme, once all the Arc-like naming conventions and macros are in place, are setforms, defcall (not in pg's Arc), and the ssyntaxes (a:b c), a.b, and a!b. Going by the examples, Rark has setforms, and its ability to unwrap data structures using function calls indicates defcall wouldn't be hard to add if it isn't there already, but I don't see anything about ssyntax. I've been meaning to download and run Rark to see if ssyntax support is actually there after all.
The abstract "Scheme" I'm talking about might have a certain advantage over Racket, but I might just be doing it wrong: Is possible to write a macro and use it in the same file? I've had to break my utilities into three files just to have multiple layers of macros. I'm interested in seeing whether Rark makes this any easier.
"If you looked at it closely, I made many changes in the spirit of getting something running that PG (and you!) would likely not agree with."
I didn't see the repo while it was online, but I've always looked forward to Arc (or at least Anarki) becoming usable as a Racket language at some point, even if it were an awkward fit. Just how disagreeable are we talking, here? :)