The problem is, even though it's the same administration, there is no legal groundwork available to refuse one a registration. While the BCAA can withhold an AOC they cannot do so for a registration unless they have proof that the plane has legal issues or has no insurance for example (and even the insurance is a non issue since you technically only have to provide a proof of insurance on the day of registration, nobody can stop you to cancel the insurance right after registration to save funds (i'm pretty sure this won't be the case here since Air Belgium doesn't own the planes and are probably contractually obligated to insure the planes (or maybe the lessor provides the insurance, which is also possible)). Few reasons can stop a registration (non-limitative and small example list): Invalid or incomplete paperwork (which can be something simple but can also be very extensive), it has been marked stolen, there's been a lien placed on it, etc... If it's all clear and nothing is wrong with it and you pay the fees for the registration the BCAA has to register the plane even if they know for example that they won't issue an AOC to the airline (which I hope won't be the case).
Anyway, we'll still have to wait and see. Personally I don't think it's the smartest move they could do, I have the impression that the registration of the planes and the press briefing earlier this month was solely to put pressure on the BCAA. While they might eventually grand an AOC (and probably will do so), the BCAA can make it very hard for AB to operate with surprise inspections and being more though than they normally would be. The BCAA has the power to make an airline very miserable if they want to... It's a federal government agency that you want to keep happy and certainly in these days when there's a lot of scrutiny, back in the days you could get away with it by offering some incentives (free flights, some gifts, etc...) but today it's being frowned upon (at least). I've enjoyed the good old days when I needed some paperwork from certain federal government agencies and a few cases of decent wine or something did the trick and fast tracked everything but slowly this all changed. So today it's more to keep the agency in question happy by not being to annoying than giving incentives