One possibility to keep being flexible would be to redefine \ang with an xparse argument signature like the following {omo} to add the optional unit in the third argument. This macro then could easily be redefined.
Unfortunately this would either include a call to \SI and thus loosing the possibilities of anglular minutes and angular seconds of the original \ang macro or include a call to the old \ang macro. The latter is to my knowledge not possible without code duplication, as neither the \let nor the \LetLtxMacro commands can cope with macros defined by xparse (which is the case for \ang defined in siunitx).
As a workaround it is possible to use the following definition until there is a better solution:
\NewDocumentCommand{\angsi}{omom}{%
\ang[#1]{#2}\si[#3]{#4}%
}
Then it is possible to write
\angsi{42}{\per\second}
or
\angsi{1;2;3}{\per\second}
and add spacing as \, to the macro definition later, if that should be needed.
Update:
I contacted Joseph Wright (siunitx package author) and he proposes
<num>°\,s^{-1}
according to the NIST standard:
There is a space between the numerical value and unit symbol, even
when the value is used in an adjectival sense, except in the case
of superscript units for plane angle.
So that's the way to go, which can be achieved by
\NewDocumentCommand{\angsi}{omom}{%
\ang[#1]{#2}\,\si[#3]{#4}%
}
Define \wn slightly different using siunitx' \tothe or \raiseto:
\DeclareSIUnit\wn{\raiseto{-1}\cm}
or
\DeclareSIUnit\wn{\cm\tothe{-1}}
A complete example:
\documentclass{article}
\usepackage{siunitx}
\DeclareSIUnit\wn{\cm\tothe{-1}}
\sisetup{per-mode=symbol}
\begin{document}
\si{\wn\per\kelvin}
\end{document}
Best Answer
The 'unit parser' in
siunitxonly operates on 'symbolic' units such aswhere the behaviour of
\peris selectable. For the 'literal' inputparsing is not possible ('literal' input could be anything), and instead
\perswitches to simply inserting a/. Thus while the 'interpreted' mode requires slightly more typing, as it is more 'programmable' it's the recommended approach.