…
Well, I’ve determined that adding an auxiliary input to a 2005 Subaru Forester XT (well, the Subaru six-disc changer) is possible, but will be complicated. It appears that with the info and software on this page I could build just such an input.
It would then be possible to have a single cable running to the iPod’s dock connector to provide power and take a line-out. This would then be fed into the head unit and selected with (I believe) the AUX button. Or it might be whatever button selected an external CD changer…
There is also some interesting info on interfacing to Panasonic head units – essentially emulating CD changers – to be found here, but I think the first page is more along the lines of what I’m looking for.
So, I think for around US$75 or so, plus a bit of work, I could have a module which would connect into the head unit’s power and CD changer connections and happily allow my iPod to play. Then maybe another $30 or so to get everything mounted nicely…

