I found bug 784028 which suggests the append behaviour is "fixed" in 2012a.
But many questions remain. Is it now impossible to save uncompressed data with -v7.3? (since previously the only way was by exploiting the append bug).
What is the criteria for whether a variable is compressed? There seems to be some size test - small variables are not compressed.
Some tedious trial and error using the third party HDFView tool apparently indicates that if the variable size is > 10000 bytes compression if used and not otherwise.