I too need to process CSV files with first line schemas. Unfortunately the schemas can vary from file to file too.
Not seeing anything out there I got to work and coded up a subclass of logstash-input-file that adds CSV parsing with a first-line-schema mode. The basic CSV processing is largely borrowed from logstash-filter-csv.
Code is at https://github.com/jweite/logstash-input-csvfile. It's strictly alpha at this stage, but I'd be interested in having it considered for submission.
PS, while I initially considered enhancing logstash-filter-csv I ultimately concluded that the only 100% reliable way to restart stream processing mid-file was to re-read the file's schema row, something that only the file input plugin can always do.