I appreciate the recommendation, but as it happens I have already solved the estimated issue (mostly by just preparing a full return with estimated next year's figures, which while time-heavy is going to be more accurate). And for what it's worth, I suspect it wouldn't work for me - cross-border is extraordinarily complicated because of the tax treaties (indeed there is a dedicated IRS form to disclose the effect and rationale of any treaty based position), and given there is no obvious note on the documentation for it that there is cross-border support I doubt there is.

(And as I said to another commenter, I think it would be advisable in this thread for people to expressly acknowledge or disclaim any interest in or prior experience with any particular product)