The British Royal Family do seem to have a very patchy record both when it comes to attending Royal events elsewhere in Europe and in its treatment of the Belgian Royal family in particular.
Looking back at British representation, it's clear that the Queen herself has very very rarely attended coronations, weddings and funerals. Why she, as Sovereign, has chosen to do this, I don't know, but it seems that for better or worse, it is the norm. In this context, her attendance at King Baudouin's funeral must be seen as a significant mark of personal respect.
During her reign, she has very often been represented by her husband, her sister (back when she was the first adult in line to the throne) or the Prince of Wales, so basically (one of) the most senior representative(s) possible. On other occasions, the Kents have represented her, particularly when there has been a close family relationship (through Princess Marina, Duchess of Kent) and in recent years, it would seem that the Earl & Countess of Wessex have been the designated family members for such European events. The Duke of York seems to be the "man in the Middle East" & the Princess Royal seems to prefer other sorts of engagements than those involving high ceremonial.
It also strikes me that King Albert II was never awarded the Order of the Garter, making him the only reigning Sovereign/Grand Duke (Liechtenstein & Monaco excepted - and monarchs where the previous sovereign is still alive) never to have received this honour during the Queen's reign. He never made a State Visit to the UK, either, if I'm not mistaken.
If indeed no senior royal is able to make the trip (a big "if") I'm wondering if it would be worse to send a minor royal than none at all.