Olhando para o código de ruby-mode, parece que não há como configurá-lo. Uma solução alternativa está substituindo a função. Experimente o código abaixo e veja se funciona:
(defun ruby-smie-rules (kind token)
(pcase (cons kind token)
(`(:elem . basic) ruby-indent-level)
;; "foo" "bar" is the concatenation of the two strings, so the second
;; should be aligned with the first.
(`(:elem . args) (if (looking-at "\\s\"") 0))
;; (`(:after . ",") (smie-rule-separator kind))
(`(:before . ";")
(cond
((smie-rule-parent-p "def" "begin" "do" "class" "module" "for"
"while" "until" "unless"
"if" "then" "elsif" "else" "when"
"rescue" "ensure" "{")
(smie-rule-parent ruby-indent-level))
;; For (invalid) code between switch and case.
;; (if (smie-parent-p "switch") 4)
))
(`(:before . ,(or `"(" `"[" `"{"))
(cond
((and (equal token "{")
(not (smie-rule-prev-p "(" "{" "[" "," "=>" "=" "return" ";"))
(save-excursion
(forward-comment -1)
(not (eq (preceding-char) ?:))))
;; Curly block opener.
(ruby-smie--indent-to-stmt))
((smie-rule-hanging-p)
;; Treat purely syntactic block-constructs as being part of their parent,
;; when the opening token is hanging and the parent is not an
;; open-paren.
(cond
((eq (car (smie-indent--parent)) t) nil)
;; When after `.', let's always de-indent,
;; because when `.' is inside the line, the
;; additional indentation from it looks out of place.
((smie-rule-parent-p ".")
;; Traverse up the call chain until the parent is not `.',
;; or `.' at indentation, or at eol.
(while (and (not (ruby-smie--bosp))
(equal (nth 2 (smie-backward-sexp ".")) ".")
(not (ruby-smie--bosp)))
(forward-char -1))
(smie-indent-virtual))
(t (smie-rule-parent))))))
(`(:after . ,(or `"(" "[" "{"))
;; FIXME: Shouldn't this be the default behavior of
;; `smie-indent-after-keyword'?
(save-excursion
(smie-rule-parent)))
(`(:before . " @ ")
(save-excursion
(skip-chars-forward " \t")
(cons 'column (current-column))))
(`(:before . "do") (ruby-smie--indent-to-stmt))
(`(:before . ".")
(if (smie-rule-sibling-p)
(and ruby-align-chained-calls 0)
(smie-backward-sexp ".")
(cons 'column (+ (current-column)
ruby-indent-level))))
(`(:before . ,(or `"else" `"then" `"elsif" `"rescue" `"ensure"))
(smie-rule-parent))
(`(:before . "when")
;; Align to the previous `when', but look up the virtual
;; indentation of `case'.
(if (smie-rule-sibling-p) 0 (smie-rule-parent)))
(`(:after . ,(or "=" "+" "-" "*" "/" "&&" "||" "%" "**" "^" "&"
"<=>" ">" "<" ">=" "<=" "==" "===" "!=" "<<" ">>"
"+=" "-=" "*=" "/=" "%=" "**=" "&=" "|=" "^=" "|"
"<<=" ">>=" "&&=" "||=" "and" "or"))
(and (smie-rule-parent-p ";" nil)
(smie-indent--hanging-p)
ruby-indent-level))
(`(:after . ,(or "?" ":")) ruby-indent-level)
(`(:before . ,(guard (memq (intern-soft token) ruby-alignable-keywords)))
(when (not (ruby--at-indentation-p))
(if (ruby-smie--indent-to-stmt-p token)
(ruby-smie--indent-to-stmt)
(cons 'column (current-column)))))
(`(:before . "iuwu-mod")
(smie-rule-parent ruby-indent-level))
))
A parte que editei está logo abaixo do FIXME, alterando para (smie-rule-parent), e pareceu funcionar para mim.