Personally, I think it's debatable. Right up until a law says "including signal" there's a basis to say "we can't"
In the final analysis, just don't put signal infrastructure in Belgium. On handset, it's the individuals obligation to unlock not the service facilitator.
Banning cryptography and encrypted at rest and undecryptable content held by servers, and as I said, if the DC is outside Belgium there's nothing to seize inside their warrant distance except the perps own phone, which is already subject to law.
In the final analysis, just don't put signal infrastructure in Belgium. On handset, it's the individuals obligation to unlock not the service facilitator.