This function has just recently been added :)
But i'm too much used to use:
/target <axy> [x=1,2 ; y=0,1,2,3,4,5]
/target <a10> will target the party leader of the topmost party.
/target <a25> will target the last person of the middle party.
I always forget whether to start on 0 or 1, but I did find an alternative technique. It's probably not as good as some of the other techniques listed as it is somewhat slower.
Once you target someone in your alliance, they are added to the autotranslate. So if you just spam <tab> to get everyone in there, you can do
/ta Fo<tab><up arrow><enter><enter>
and it will target Foobar in your alliance.
/ta does same as /target but fewer keystrokes
Fo is first few letters of player's name
<tab> open autotranslate
<up arrow> wraps around to bottom entry. It puts the people at the bottom
<enter> to select autotranslate
<enter> to finish target command