I think the answer is no. You cannot pass additional variables in the callback string 'callback_sku_check'.
That has caught me out a few times when I think 'I can do a callback to validate that' and then find I can't.
The only thing you can do is access the post array directly in the callback. However in your case you will be accessing the entire array, and would have to identify which term the callback is currently being called on, which again I do not know how you would do that.
To do whatever you needed to check here, I would do this as a post validation logic check. So you have passed validation, so all the data is valid, now you need to do some further checks before proceeding, but those fails are not passed as validation errors, but as a normmal message to the user, displayed in your form however you would normally display a user message. (I hope that makes sense).
As a for instance, a login might pass validation for the form sending an email and a password, but would fail the logic check done post validation that the user is actually registered, so you would pass back to the login form with the error message 'You are not yet registered'.
Hope that helps,
Paul.