I noticed that, when generating the doc, the load_only flag is not respected, and fields marked as such will appear in the description of the output schema.

Serialisation itself seems to be done correctly and the field does not seem to be returned by the query.

From what I can see, this is due to the (expected) behaviour of fields2jsonschema, which does not exclude load_only fields:
marshmallow-code/apispec#119

I am not too sure why serialisation works (wouldn't it get the same schema?)… But in any case, do you have any opinion on what would be the best way to fix this?

I guess one option is to create a DumpOnlySchema mixin similar to what is being done by PostFormParameters for dump_only fields? Any better way?