Ok, you meant the userblock at the beginning of the file. Yes, I would know how to access that (I used it before to embed an HDF5 file into an HTTP stream, allowing the same file to be read from disk or from an internet socket via a webbrowser).
However, for a compression filter such a filter-specific dictionary should be “closer” to the data sets, because it would depend on the category of data. For instance, I would imagine using a different compression dictionary for RGB data than for floating-point data. Maybe even different dictionaries for RGB data describing images of lakes than for RGB data describing images of mountains. So a better place for such a dictionary would be an attribute on the datasets, or even better, an attribute on the named type that is used for a dataset, such that multiple datasets can use the same dictionary, but not all datasets use the same. Just, how can such information be passed to a filter?
For the current issue of the zstd filter, such a feature seems not really needed, since the dictionary is apparently only useful for small data. We want to think in big data anyway. But it could be useful for AI-based filters that require some training data. I don’t have experience with those yet, but it seems there are some in active development.