When we were at Walsrode the Kagus' enclosures were just aviaries like any other. The birds were clearly doing very well though - we saw around a dozen birds and they regularly breed.
The other European zoo where these can be seen is Wuppertal, where they are also housed in pretty standard bird accommodation. I haven't seen Berlin's yet - missed them by about a year.
I saw a pair of kagu at the menagerie of the jardin des plantes in paris on a sunny morning in March, 2008. They were outside in the pheasant section. My camera battery stopped and i went to the shade to change it. When i returned the kagu were back in the indoor shelter and did not venture out again.