Hey Ronny,
A good place to look for any web API questions is the documentation. Here's the one for limelm.pkey.advancedSearch.
Is it possible to get the value of a custom license field returned with this call?
Yes, you can either search by custom fields (see: feature_name[], feature_value[], feature_match[]), and/or you can set the advancedSearch function to return the licens fields with the product keys. See: get_features