Not in Arc. The best you get is (.g:.d:.c:a "b"), I think. You can shove "b" into ssyntax with string!b, but you need to use it as an argument somehow, and any ssyntax containing .string!b will pass the 'string function itself as an argument instead of calling it first.
Of course, if "b" were a symbol instead of a string, it would just be "a!b.c.d.g".
In Penknife, the answer to your question is actually yes: It's "q.b'a.c.d.g", where q is the string-quoting operator and ' is an operator that acts like a reverse a.b. It's easy to stump Penknife too, but I'm hoping to make a full thread about Penknife's take on infix operators in a couple of days or so.
arc> (= test (obj "a" "my a val" "b" "my b val"))
#hash(("a" . "my a val") ("b" . "my b val"))
"my a val"
[edit: actually, it would be nicer to have the percent symbol represent the spaces in the string and have some other symbol signify string handling, but I never got around to it + my scheme foo is lacking :)]