I'd say it *can* be written this way. Not that it is taught to be written so, or even encouraged, but everyone who reads it will understand what is meant.
I do not believe this is a remnant from the Kurrent or Sütterlin, because there the line denoted something completely different, as has already been pointed out. Also these older scripts are not intelligeble to most younger persons, even in German speaking areas. 
Instead this is a very common way of writing these diacritic signs in other languages using these. In Swedish we *were taught* to write the dots as a curved line in *Handwriting*. (Nowadays handwiting isn't even taught anymore.) The same goes for Danish and Norwegian. I therefore think it might have more to do with a common trait than with a supposed link to older ways of writing.
The fact that writing this way in German could cause a confusion, because of the old n/u distinction function, should make it less approprate though.