Think I got an OK implementation. You can read it here[0]. I also
changed the syntax slightly in the process:

(alist-values =? ALIST LVAR ...)

LVAR ::= KEY
        | (KEY)
        | (KEY DEFAULT)

(alist-let =? ALIST (LVAR ...)
   BODY ...)

LVAR ::= (VAR KEY)
        | (VAR KEY DEFAULT)

=? is the key equality test. In retrospect I don't think it makes sense
to have an optional key equality test for each key, because it has to be
"compatible" with all keys of the alist. Instead, the equality test is
now mandatory and will be used for all key-based lookups.

For some dictionary types other than alists requiring the equality test
may not make sense (e.g. SRFI 69 hash tables). So the *-let syntax may
be slightly different, but very little change to switch dictionary types
will be required.

Waiting for comments on what people think. Next (don't know when) I'll
try to use Zipheir's idea of a "binder-maker"[1].

[0]: https://git.sr.ht/~siiky/alist-let/tree/main/item/alist-let.scm
[1]: http://ix.io/3Zfd