Now that @ may begin an identifier, we should probably make some comment about unquoting, i.e. to distinguish whether ,@foo is (unquote @foo) or (unquote-splicing foo).
Since these identifiers did not exist before, I recommend the latter (that is, use unquote-splicing if ,@ is seen), and we should add a note that if you want to unquote an identifier beginning with add you need to either insert whitespace or escape the identifier, e.g. , @foo or ,|@foo|.
The good news now is that SXML is no longer invalid syntax.
The above states that maybe we treat ', @foo' as an unquoted identifier @foo, and Chez Scheme does this. Is Chez Scheme not in your test suite, or were you testing something else?
No, my test case was incorrectly stated. I retract my comment: , @foo works fine to unquote the variable @foo.
The WG voted to treat ,@foo as (unquote-splicing foo), requiring , @foo for (unquote @foo).
None of my suite of Schemes supports , @foo as equivalent to ,@foo, so I think that option is out. Users will need to write ,|@foo|, which works on all Schemes that support pipes (some do not, some treat | as an ordinary identifier character).