Generally, I think you need an "experimental" registration, rather than an ultralight (or microlight, as it is sometimes termed). The F-Pxxx registration might be the least difficult. It is no surprise that a prototype like the Verhees Delta, also Belgian based, is in that category. (
Verhees Engineering > Delta )
In this particular case, you might wish to come and have a chat at Kiewit EBZH aerdrome, there is a DR1-replica stationed. ISTR it has a PH-registration.