An external library is exactly what you said: adding an addtional Runtime through the libraries. And you can have several. I have sorted my content in different libraries to make it easier to find i e Victoria 4, Michael 4 etc.

I think there was somebody asking about a similar problem in the DAZ forums. I don't remember if there was a solutiion though.  Also IIRC there was somebody else whose external runtimes worked fine when they put them someplace in the Documents folder (Users/myusername/Documents/somefolderhere).

Also I find that I can add both a folder containing a Runtime folder and the Runtime folder itself to Poser 7 (mac, leopard, Intel). There's a folder called Runtime in my applications folder that either DAZ Studio or Carrara installed there. i tried adding that to Poser 7 and sure enough, it turns up in the list with the name "Applications". :)