Someone will correct me if I'm wrong, but I think that, as long as
your segment
1) is a subclass of HL7::Message::Segment
2) is defined in the segment namespace (for example,
HL7::Message::Segment::IN1)
then you should be able to define the segment withing your project w/
out forking the gem.
There are plenty of examples of extending Segment in
https://github.com/ruby-hl7/ruby-hl7/tree/master/lib/segments