When a verb such as "can" is modified by "only", the meaning of the sentence becomes rather negative (impossibility rather than possibility), and to me it sounds strange to use the negative tag "can't it?". 
On Google Books I find an example of an author doing it so,

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> It can only be Jagged Sky we're aiming for, can't it? (The Empty Sea, by Craig Michael Curtis) 
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But I'm not sure it's the most cautious literature. I would have said *"isn't it"*, based on meaning:

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> It can only be Jagged Sky we're aiming for, isn't it? (= isn't it Jagged Sky we're aiming for?)
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Is there a rule, and what is the actual usage?

EDIT: I understand that since "be" was present in the first example, my alternative suggestion is confusing as it seems to be related to it. A second example is:

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> Lorries can only use the main road, can't they?
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where I wonder whether one would employ this negative tag or rather a positive one, *"can they?"* (=*"can they use other roads?"*)