readline supports startup_hook.

readline_hack.rb

require 'readline'
require 'fiddle/import'

module Readline
  module LIBREADLINE
    extend Fiddle::Importer
    # このパスどうしよ
    dlload '/usr/local/opt/readline/lib/libreadline.6.2.dylib'
    RL_STARTUP_HOOK = import_symbol 'rl_startup_hook'
  end

  def self.set_startup_hook(&blk)
    ptr = LIBREADLINE::RL_STARTUP_HOOK
    cf = LIBREADLINE::bind('int startup_hook_callback()') { blk.call }
    ptr[0, Fiddle::SIZEOF_VOIDP] = [cf.to_i].pack(Fiddle::SIZEOF_VOIDP == 4 ? 'I!' : 'L!')
  end
end

loop do
  Readline.set_startup_hook do
    Readline.insert_text 'default text'
    0
  end
  buf = Readline.readline '> ', true
  puts buf
end

と言うか、 startup_hook くらい最初からサポートしてて欲しい