"My solution relies on unquote and therefore on backquote."
"More strongly, I want to argue that having syntax for quote/backquote/unquote is valuable."
I don't think that's as expressive as infix syntax. With the right kind of a.b syntax, we could write `(a b ,c) as qq.(a b uq.c), trading in some readability for the flexibility to define our own variants. Would you still want to have dedicated quasiquotation syntax then?
(Even I might answer yes sometimes. The staged fexpr system I describe at http://arclanguage.org/item?id=15868 uses unquote syntax as a way to compute during a previous stage of the command processor.)
 Penknife does something like this, but the use of structured string manipulation instead of s-expressions forces it to use a string-like escaping mechanism for unquote: qq.[a b \,c]