Is there a list of characters specifically used for western proper nouns? When learning country/place names or reading about non-Chinese people in the media I see the same characters used for similar phonemes (see these celebrities). What rule or list do the writers follow when 'coining' a hiterto untranslated name into Chinese?
No list. It would be easy to make a list because it would be all characters (point 3). In general I think these are the rules:


1) The standard (which is really based off the next point).
2) The frequency/stroke count of the character.
3) The desire to represent style.


The standard

I just mean x character has been used before, lets use it again because folks already know about it. Really just based off 2 and 3 below.

The frequency/stroke count of the character.

I feel like Starbucks is an interesting example, a phono-semantic one too. 

星巴克
xīng ba kè
star 'phonetic placer' 'phonetic placer' 


The last two characters are the most interesting because they have no meaning and are used because of their sound and frequency/stroke count. 

巴 - 697 frequency (also the first ba in the list)
克 - 493 frequency (also the first ke in the list)


http://www.zein.se/patrick/3000char.html

The desire to represent one's style.

I've never heard about a law that saws parents cannot use x character to name their children. I think such censorship would cause a revolution before youku got shutdown. So following this point theoretically any character is up for grabs.