distinguishing between cautionary accidentals and musica ficta

[Graham King]

In editions of renaissance music, both cautionary accidentals and ficta may be notated above the staff.  I would like to distinguish between these cases, using parentheses.  It is possible to do so, on a case-by-case basis, using 
>     \once \override Staff.AccidentalSuggestion.parenthesized = ##t
but I would like to use a shorthand that detects cautionary accidentals automatically.   For example, in the following code, the natural above the b should be parenthesized:
>     \version "2.19.2"
>     
>     ficta = {
>         \once \set suggestAccidentals = ##t
>         \once \accidentalStyle "dodecaphonic" % mark _all_ the ficta in the bar
>     }
>     
>     \relative c'' {
>       g4 \ficta fis g2
>       c4 \ficta b? c2
>     }
How may this be achieved?  Suggestions gratefully received...

-- Graham