The lack of a bitcoind in MacOS is quite unfortunate (and a bit mysterious). Installing it is not simple (at least not if you are an Armory install script). I have never bothered, and is using Bitcoin-Qt with Armory.
I would suggest you do the same. Bitcoin-Qt can take arguments that enable RPC, I saw that somewhere but cannot find it now (someting ugly like "open /Applications/Bitcoin-Qt.app --args args-to-bitcoinqt"). There is not way to start it hidden, but you can hide it after is has come up with this ugly command
osascript -e 'tell application "System Events" to set visible of application process "Bitcoin-Qt" to false'
I found it here:
https://discussions.apple.com/thread/1476240?start=0&tstart=0 Trying to make a programming language to resemble a natural language results in something that is neither natural for programmers nor for nonprogrammers. Applescript looks awful. It will not hide the splash screen of Bitcoin-Qt, but it will hide the app as soon as the splash screen goes away, even if executed while the splash screen is there (just tried it out of curiosity).
I guess that the best alternative for Mac users is to use Bitcoin-Qt in this way. The icon will still appear in the dock, however.
At one point, bitcoind came with Bitcoin.app, but not anymore for some reason I don't know. That's one of the reasons I made my brew formula. `brew install bitcoind` is pretty easy. I'm going to make the formula also create a plist for launchd so you can configure bitcoind to start on login if you want by just making a symlink.
Is starting armory on login something that would be desired? Brew makes it really easy and totally optional.
If you have armory searching for bitcoind on the PATH for linux (Please don't use hard coded paths), you might as well have the same check happen for osx. If they have used my brew formula or compiled it themselves, it will just work. If that search fails, check for /Applications/Bitcoin-Qt.app and launch it with whatever crazy flags make it hidden. Then if that fails, just tell the user "Armory couldn't find bitcoind or bitcoin-qt installed on your system." and then have system specific links if possible and a link to bitcoin.org if not.
I think that having the user install bitcoind/qt and armory smart enough to find it is plenty fine and is definitely going to be exponentially easier. There are far too many possible system setups. Any code you come up with for trying to install packages on all the different systems is going to be so complicated that I don't think its worth it. Just ubuntu alone has a couple ways (self-compiled, PPA, downloaded from bitcoin.org)