In the end a feature will just be a numerical value.
How do you plan to use synonyms in a field to generate a numerical feature ?

Are you planning to define a binary feature for a field, in case there is a
match on the synonyms ?
Or a feature which contains a score for a query ( with synonyms expansion) ?

I would start from the SolrFeature, let's assume the "title" field has a
field type that includes synonyms ( query time) :

{
    "store" : "featureStore",
    "name" : "hasTitleMatch",
    "class" : "org.apache.solr.ltr.feature.SolrFeature",
    "params" : {
      "fq": [ "{!field f=title}${query}" ]
    }

Query time analysis will be applied and synonyms expanded.
So the feature will have a value , which is the score returned for the query
and the document ( under scoring) .
You can play with that and design the feature that best fit your idea.

Regards
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Alessandro Benedetti
Search Consultant, R&D Software Engineer, Director
Sease Ltd. - www.sease.io
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