This is a simple example of how to create a redmine plugin, which adds a new custom field format to allow dynamic values to be used as options for custom fields

001_add_author_to_project.rb

# redmine/plugins/project_author/db/migrate/001_add_author_to_project.rb

class AddAuthorToProject < ActiveRecord::Migration
  def up
    ProjectCustomField.create(
      name: 'Author',
      field_format: 'all_users',
      default_value: nil,
      is_required: false,
      editable: true,
      visible: true
    )
  end
 
  def down
    ProjectCustomField.where(name: 'Author').first.try :destroy
  end
end

all_users_field_format.rb

# redmine/plugins/project_author/app/models/all_users_field_format.rb

class AllUsersFieldFormat < Redmine::FieldFormat::List
 
  def possible_values_options(custom_field, object=nil)
    User.all.collect { |u| [ u.to_s, u.id.to_s ] }
  end
 
end

init.rb

Redmine::Plugin.register :project_author do
  name 'Project Author plugin'
  author 'Jan Svoboda'
  description 'This is a plugin for Redmine to specify author for each project'
  version '0.0.1'
  author_url 'https://github.com/svoboda-jan'
end
 
Redmine::FieldFormat.add 'all_users', AllUsersFieldFormat