Hi!
...being aware that flying VFR below FL100 across Germany in a jet is not very sensible...
You don't need to stay below FL100 for this kind of flight! You can ask for (and will be granted) clearance to climb through airspace C between FL100 and FL200 and get an IFR pickup there (for some obscure reason, the airspace above FL200 is considered to be ash-free enough for instrument flying). The same procedure in reverse then applies for the descent. I talked with DFS myself a few hours ago.
I was faced with the same decision as you this morning and "delegated" it to my boss, since he is the one who is going to pay for the engine overhaul. Very sensibly, he decided to postpone the flight for a few days.
Regards, max