This trip was finally done here.
The VFR sections were done with some issues, which would be well known to any pilot who has done any significant VFR flying. In particular, while French ATC will generally sort out CAS access for you, they do not coordinate on restricted or prohibited areas and will happily leave you to bust them. And the same will happen if flying VFR from Switzerland to France across the SE border which is lined with these restricted areas and you will bust one of these immediately you cross the border; Swiss ATC won’t stop you doing it (they may warn you but they are not required to).
Whereas if you fly IFR none of this is an issue. So it pays to get an IFR clearance as soon as possible, once the VFR flying option is no longer needed.
I would fly at FL080 if there is any headwind, or FL150/160 if there is any tailwind. But FL080 in IMC = ICE especially at this time of the year, and this is an issue for legs filed VFR to get shortcuts. The tactic for IMC would then be to ask if a change to IFR would still enable that routing.
Don’t plan on over FL115, unless you have the phone number of some Paris Control manager. Edit : maybe Lille can get you a higher clearance given with how little traffic they have.
Why would he call anybody when flying IFR?
Edit: I missed few posts above “VFR” :)
I will put 99% money on it, Lille FIS West (it’s ATC for Lille approach & also Lille East now) will give you VFR clearance above FL120 (subject to LFCBA1A status which also affect IFR routing to CMB VOR anyway), 1% worst case it’s OCAS up to FL115 
Don’t plan on over FL115, unless you have the phone number of some Paris Control manager.
Otherwise, no worries at all. You will just be with Lille Info.
The best would be early morning on a weekend : no traffic nor military training, which is frequent east of Reims.
Edit : maybe Lille can get you a higher clearance given with how little traffic they have.
Can anyone from France comment on whether KUNAV-BILGO is OK VFR and possibly whether it can be done above FL120.
For VFR I can’t see anything obvious on the charts that would be a problem.
Indeed; an excellent Q.
What concerns me is running out of airport opening time, and having to stay overnight, which would complicate things (and my camping gear is not good enough for Italy in the winter ).
OTOH if stopping for fuel, one could fly both ways flat out.
I don’t understand why you don’t just land in Aosta to refuel? There’s a specific exemption for quarantine for flight crew in the UK.
The ones east of Paris seem to be 24/7, at all levels I have ever flown at, and the Paris TMA is a no-go below FL200. So, east of Paris one always flies within the same corridor, with practically no deviations (within maybe 1-2nm). I have had exceptions, when I really pushed (due to headwind etc) but they were very unusual.
What also interests me is whether KUNAV-BILGO is OK VFR and whether it can be done above FL120. Much of France doesn’t allow VFR above FL120 (Class D) by notam. Normally I would fly at FL100 but if I get IMC < 0 then I will be looking at a climb to VMC. I’ve been looking at the VFR chart and the typical route I get (IFR) runs DJL-VATRI-REM and that does actually clip the Paris TMA (7 and 9) and just clips the military areas to the east of it.
