Allies automatically have Sorcery and at least one spell they can cast. There is nothing in canon to suggest that an ally doesn't have the same TN penalty for sustaining a spell as any other spellcaster, although there might be a suggestion that the ally can sustain a spell for the caster at the same time as doing its own thing.

Can an ally bond a sustaining focus? More drastically, can an ally be conceived to inhabit a sustaining focus, and be able to take full advantage of it? Heck, can one of the forms of an ally be linked directly to a sustaining focus, with the intent that the ally should be able to use it? (I draw the line at it being that focus, since in a way it already is. I'm talking about linking an already existing focus in with the ally's formula.)

Would there be any limit on the rating of such a focus, while still being fully usable by the ally? Could an ally become subject to focus addiction?

I would allow it. Foci have enough disadvantages that it would add very interesting twists both working for and against such an ally. I would enforce "focus addiction" for an ally spirit. I might give it different fluff by calling it astral entrapment or some such. Perhaps a sprit bonding to foci like that risk becoming a "demon in a bottle" with high force foci. Maybe not perfectly thought out, but my .2.

[edit] Of course, since allies can't earn karma, the magician would have to do this all with a ritual of change or some such like you said. [/edit]

It would have to be an inhabited homonculus/sustaining combination focus at the very least, since in order to inhabit a focus, an ally must use its inhabiting power which is limited to animal form and a homonculus. Allies follow any and all rules for regular spellcasting, so it takes the +2 modifier for sustaining. When they go free, the rules change, but that's another story.