Microsoft Sharepoint has the wonderful feature called Slide Libraries which does exactly what you want:
1 master slide on a server, which you 'import' into each of your 20 PPT's to become linked duplicates.
You can edit your master slide on the library whenever you want, but for each of the 20 PPTs you will still have to open them and press a button to have them 'pick-up' the new version of your linked slides...

Only drawback is: you need the Sharepoint server.