I finally cracked and wrote my own naive Things database syncer.

#!/usr/bin/ruby

require 'rubygems'
require 'net/ssh'
require 'net/scp'

thingsPath = "HOME/Library/Application\ Support/Cultured\ Code/Things/Database.xml"
getTimestamp = "ls -lT '#{thingsPath}' | cut -f9-13 -d' '"
getThings = "ps waux | grep [T]hings"
quitThings = "osascript -e 'tell application \"Things\"\nquit\nend tell'"
openThings = "open -a Things"

Net::SSH.start("YOUR-REMOTE-HOST.ORG", "USERNAME") do |ssh|

  home = ssh.exec!("echo $HOME").chomp

  remotePath = thingsPath.gsub(/HOME/, home)
  remoteGetTimestamp = getTimestamp.gsub(/HOME/, home)
  remoteTimestamp = DateTime.parse(ssh.exec!(remoteGetTimestamp))

  home = ENV["HOME"]
  localPath = thingsPath.gsub(/HOME/, home)
  localGetTimestamp = getTimestamp.gsub(/HOME/, home)
  localTimestamp = DateTime.parse(`#{localGetTimestamp}`)

  if remoteTimestamp > localTimestamp
    puts "Remote host has more recent Things database."
    
    if `#{getThings}` != ""
      puts "Quitting local Things."
      system(quitThings)
    end
  
    ssh.scp.download!(remotePath.gsub(/ /, "\\ "), localPath)
    puts "Received file."
    
    puts "Re-opening local Things."
    system(openThings)
  else
    puts "Remote host has out of date Things database"
    
    if not ssh.exec!(getThings).nil?
      puts "Quitting remote Things."
      ssh.exec!(quitThings)
    end

    ssh.scp.upload!(localPath, remotePath.gsub(/ /, "\\ "))
    puts "Transmitted file."
    
    puts "Re-opening remote Things."
    ssh.exec!(openThings)
  end
end