Support custom hash tables?

https://github.com/metawilm/cl-custom-hash-table provides us with a portable way to define custom hash tables.
These are still of the `hash-table` type and thus are serialized by prevalence like any other hash table, but with the `test` set to the custom test.
The new `hash-function`, however, is not serialized.

This causes problems on deserialization because the custom test alone is not enough to make the custom hash table: the hash function must be provided.
Otherwise, the following condition is raised:

```lisp
Unknown :TEST for MAKE-HASH-TABLE: MY-TEST-FUNCTION
```

One way to work around this would be to serialize the hash-function name. Then, to deserialize it, the library would use cl-custom-hash-table.
The problem is that I can't find a way to access the hash-function used for a hash-table value. Any idea?