I’ve just seen your post and since you’re in the same position I am in, here is what I do at the moment to work around the problem until it’s solved.
You can use your survey data and copy a single point, ideally in the centre of the location you’re surveying into this calculator: http://gibs.bkg.bund.de/geoid/gscomp.php?p=g
I then take the exported CSV file and use Excel to substract the “Quasigeoidhöhe”, which you will get from the website above, from all my heights of my survey. If you use the “special paste” / “Inhalte einfügen” command you can do this easily. You copy the “Quasigeoidhöhe” and highlight all your heights in Excel, then use “Inhalte einfügen” / “special paste” and use the substract option.
I hope this helps for the moment. It obviously only works for a smallish area. If you’re surveying broader areas you will have to do multiple centres to keep the deviation at a low level.
And just to be fair, the link above was given to me by my supplier laserscanning-europe.com, when I was looking for a workaround too.