They are both widespread in Belgium.
My advice would be to look at:
Waarnemingen.be wil iedereen in staat stellen om waarnemingen op te slaan en te delen, om zo de kennis van de natuur te vergroten.
waarnemingen.be
and
Waarnemingen.be wil iedereen in staat stellen om waarnemingen op te slaan en te delen, om zo de kennis van de natuur te vergroten.
waarnemingen.be
and then zoom in on the area where you'll be staying (or do you want to make it a day trip?)
If you click on the atlas squares you will see individual locations.
With some comparing of maps,
e.g. Domein Vordenstein near Schoten, het
Vierkensbroek near Averbode or het
Meerdaalbos/
Mollendaalbos near St Joris-Weert (Meerdaalbos better for Black, Mollendaalbos better for Middle Spotted) look like good locations for both, but there must be dozens and quite probably better ones... They are also regularly seen in the Forêt de Soignes just south of Watermael-Boisfort (Brussels), but I assume this is a popular place to visit so they may not be
that common. Let's hope a Belgian member can give better advice!
Middle Spotted Woodpeckers can be quite vocal early morning, but their most common call can easily be confused with Great Spotted Woodpecker. On a nice February morning you may have a good chance of hearing their jay-like "song" as well.
Black Woodpecker can be a bit hit or miss as they have large territories but wandering around should get you within hearing distance of one calling. They have three very different calls which they like to use.