But modifiers can be included within a string or qr regex definition (and IMHO, should only be used in this way in the case of the /i modifier).

Indeed they can, and that's actually what stringification of qred regexes does, I was thinking about context-dependant changes, like use re. You could still write the whole set of modifiers in the string to be sure to get what you want no matter what the context is though.