+1 to this. There would be virtually no "inlay" (ie additional fabric folded inside the lapels) so there's nothing to let out to make the lapels wider.Not possible without an ugly seam
Therefore, the only way to widen the lapels would be to add an additional strip of fabric around the outside of the lapels. From memory, this was the fashion for a while in the 18th century, so you could always go for a somewhat steampunk/retro vibe!
Alternatively, I suppose that you could always try steaming or ironing the lapels to change the shape and make them wider, but really the only way to make that work would be to lower the buttoning point and so the jacket's balance would be off and it would look odd.
In other words, I can't see it working.