The fonts that I have downloaded are the Charis Sil (can't see any phonetic characters in the 'Symbols') It is also pretty easy to make a custom keyboard layout if you want. Yes, you should download and install that and see if it does what you need. The documents can be very long, so perhaps the keyboard layout which you mention might be the best solution that I am going to find for my system? I need to be able to add, word by word, the Italian phonetic symbols above the Italian text which I then translate into English. That sounds like the obsolete non-unicode system I mentioned, which ideally nobody should be using these days.
On the PC I have been able to type IPA symbols directly from the keyboard by means of fonts downloaded from IPA International.
In the Font Variation pane you should be able to see a good number of fonts that have these. Go to Character Viewer (Edit > Special Characters), View=Code Tables, Tab=Unicode, 00000250 IPA Extensions. I have not yet found the IPA stuff within Mac0S X 10.6.