There isn't a solution that meets all your needs. If you burn the MP3 disc in iTunes then inserting it should launch iTunes. But I don't know if it does that with other computers or just the one that burned the disc. As you've seen, burning a MP3 disc with Toast (or other application) isn't recognized that way by iTunes. The Mac sees it as a ISO9660-formatted data disc (which is true). The data just happens to be mp3 files. If you go Here
you can download an applescript from dougscripts.com that automates the creation of a new iTunes playlist containing all the tracks on a mp3 disc. This is run from the iTunes Scripts menu (just to the left of the Help menu). However, this doesn't solve your desire to have the disc recognized automatically on any computer in which it is inserted.
The applescript doesn't actually save much time or effort. You can simply choose to create a New Playlist in iTunes and drag over the icon from the mp3 disc to the playlist window. All the tracks are quickly entered. But maybe you don't want the playlist to remain after you eject the disc. Sigh.
The other problem with mp3 discs is they are alphanumeric so the track order may not end up to be what you want.
So that brings us back to the audio CD format which has the problem with no tags. As you mentioned, you can upload info to Gracenote (using either Toast or iTunes) so that it is automatically downloaded whenever the disc is inserted for the first time in any computer connected to the Internet at the time the disc is inserted. That may be your best bet.
By the way, there is another dougscript called CD Text to cdinfo which automatically transfers the CD Text information burned by Toast (which the Mac ignores) to the cdinfo database file with iTunes reads and manages. If you are in control of the other computers playing this audio CD you could add that applescript to each computer's iTunes and quickly add the info from the disc when it is first inserted and have no need for Gracenote. After the first insertion iTunes finds the info in its cdinfo file and all is well.