gistfile1.rb

# In Jekyll core...

module Jekyll
	module Filters
		module Custom
			class << self
				attr_accessor :extensions
			end

			@@extensions = []

			def self.extended(base)
				@@extensions << base
			end
		end
	end
end

# Definition file...
module Jekyll
	module Filters
		class MyFilter
			extend Jekyll::Filters::Custom
			... methods ...
		end
	end
end

#Later....
Jekyll.something(Jekyll::Filters::Custom.extensions)