The holder of a (now expired) Belgian ultralight licence, I fully concur with GtE (except for the nuance that the crux is not in the plane being sub-EASA, but rather the licence; but of course the two go hand in hand). It seems unlikely, but it never hurts to inquire. If you can manage to get someone on the phone, that is, I understand that the service is badly understaffed.
I do have a vague memory though, that at one time the mandatory 50 hours for getting a new PPL could be reduced to 40 if one had an ultralight licence.
@GtE below: thanks, G., I am sure your memory is better than mine
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