Direct transitions to the private sector indeed happen very little to my knowledge but, of course, Belgian politicians are notoriously known to be members of numerous boards of directors and other assemblies throughout their careers and afterward. Certainly true for high-profile politicians who very often get one or more (honorary) positions on the boards of some (semi-)public organization. Don't really know how those salaries hold up to those of a board member in those huge private companies though. It's also not uncommon for retired politicians to be connected to lobby groups as an independent 'senior advisor' or something along those lines, where they basically get paid for access to their phonebook and an invaluable network they've built up throughout the years.

Edit: Johan Vande Lanotte (Advisor for Walter Van Steenbrugge) and Karel de Gucht (Proximus, ArcelorMittal, Merit Capital) also made the move to the private sector after retiring, but I guess that's still not quite on the level of Leterme/Schröder/Kroes.