If I understand you correctly, there are two ways, in which this
could/should be solved on package level:
1. siunitx gets an option / command whatever, which does approximately:
\ifxetex\input{other file which can include suitable unicode symbols}\fi
2. a new package "xesiunitx" is created, which does approximately:
\usepackage{siunitx}
\sisetup{definitions using suitable unicode symbols, depending on
package option}
or
\usepackage{siunitx}
\testiffonthassymbols
\sisetup{definitions using suitable unicode symbols}
\else
\sisetup{some other helpful definitions}
\fi
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