The wind will rip it off at the first opportunity.
High speed air is as unforgiving as a curb. Observe the struts and turnbuckles on a time attack car just to get a little adjustment.
A backward leaning air dam will increase underbody air pressure. A forward leaning air dam will decrease underbody air pressure. The extreme is the diverter on the bottom of the air dam on those time attack cars.
If the concern is ground strikes, why not a passive, hinged air dam with a roller on the bottom?
http://www.carscoops.com/2016/01/its-been-70-years-since-first-vw-beetle.html
Like this on a Cal Looker instead of the KdF-Wagen.