[exim] Setting primary_hostname via command?

Is there any way I can set the value of a main config setting like
primary_hostname to the output of a system command? I've tried

primary_hostname = ${run{<command>}}

of course, but that doesn't work; it looks like the string isn't
expanded, so the value is literally set to ${run{<command>}}.

Actually, one of the things that's always confused me most about the
exim config setup is the whole string expansion mechanism...

- Toralf