What is the easiest way to associate an utxo to a specific account?
The long and short of it is that you can't associate UTXOs with accounts, you can only associate addresses. From the bitcoin wiki accounts page:


  When you receive bitcoins, they are always assigned to one of your accounts, and you can change which account is credited based on which bitcoin address receives the coins, just like you tell a bank teller which account to credit when you deposit cash in your bank. However, sending bitcoins is like withdrawing cash from the bank; the coins that are sent out and debited from an account are almost always not the same coins that were deposited into that account.


If you haven't already, you should see this post about why the accounts portion of the tab isn't really scalable. 


  I thought I'd be able to use getaccount with the returned corresponding addresses, but I'm returned an empty string. Is that because the transaction originated from the wallet 'global' account?


Yes, that is exactly, why. The default account is named "". 

So, you can associate UTXOs with accounts through associating addresses with accounts, but when spending, there's no guarantee about where you're spending from.

As a possible work around, you might look at using this rpc call:

listreceivedbyaddress ( minconf includeempty )