|Simply put, the drills for the CEREC milling unit is positioned 90 degree to either sides of the milling sprue. Since there's no drill directly facing the sprue, you won't be able to mill the innards of the coping if the tooth is milled the long way.
There are however four different ways that might help to get around your problem.
First, there is a type of block called C14L; it's only available in multi format. With this block the longer side I think is 16mm.
Second, if you have MCXL with 4.0, then you may be able to rotate your restoration within the block and can mill it at an angle that might just be long enough for you.
Third, there's a block call C40, it's a bridge block, and available with eMax only. Yes you will be wasting a lot of material and a little bit of money, heck, you can still claim that you can do a 1-visit restoration.
Lastly, if you can do eMax, then I'm certain that you have an oven and eMax stain, glaze and build up kit. Then simply move your restoration to mill the cervical area and sacrafice the incisal edge, once milled, just simply add a layer or 2 with the eMax add-on powder, then stain and glaze to perfection.
Personally, I would go for the third choice and waste a bridge block rather than toying around with adding the incisal edge.