Hi @EduardDerks.,
@Jed_Campbell 's suggestion to add this to the wish list is a great idea, please do that.
Also, there is a slight modification to @Jed_Campbell's suggestion that might get you slightly closer, which is to omit Current Window(), and start with the bare put-to operator ( i.e., << ), which tells Notebook to emit the resulting display box to the block output directly, rather than at the bottom of the window.
<< Append (
Expr As Picture( Expr( (-b + Sqrt( b ^ 2 - 4 * a * c )) / (2 * a) ) )
)

I hope this helps!
jules