Extending the idea of the polymorphic '+ on lists to include tables suggests what CatDancer proposed in the original post.

So I wrote up the following sketchy hacks of '+ for tables:

  ; arc3/tab-tests.arc

  (def table+ tbs
    (listtab (apply +
                    (map tablist
                         (rev tbs)))))
  ;; Conceptually transparent, but try to avoid
  ;; thinking about the wastefulness...

  (def table+-better tbs
    (w/table tb-new
      (each tb (rev tbs)
        (ontable k v tb
          (= (tb-new k) v)))))
  ;; Much better! Has a nice profile too :)

  ;; The rest is a "throw-away" test for the code
  ;; (require 'human-at-monitor-to-compare-tests)
  (with (ta (obj a 'ta
                 b 'ta)
         tb (obj b 'tb
                 c 'tb)
         tc (obj c 'tc
                 a 'tc))
    (map prn `("Consecutive lines should be identical:"
               ,(table+ ta tb tc)
               ,(table+-better ta tb tc)
               ,(table+ tb tc ta)
               ,(table+-better tb tc ta)
               ,(table+ tc ta tb)
               ,(table+-better tc ta tb))))

  ; Arc REPL
  Use (quit) to quit, (tl) to return here after an interrupt.
  arc> (load "tab-tests.arc")
  Consecutive lines should be identical:
  #hash((a . ta) (b . ta) (c . tb))
  #hash((a . ta) (b . ta) (c . tb))
  #hash((a . tc) (b . tb) (c . tb))
  #hash((a . tc) (b . tb) (c . tb))
  #hash((a . tc) (b . ta) (c . tc))
  #hash((a . tc) (b . ta) (c . tc))
  nil

  (xdef + (lambda args
             (cond ((null? args) 0)
                   ((all string? args) 
                    (apply string-append args))
                   ((all arc-list? args) 
                    (ac-niltree (apply append (map ar-nil-terminate args))))
                   (#t (apply + args)))))