Since the mail flow disclaimer is not an option, there are three native ways to implement such signatures:
- Deploy signatures via VBScript, which would require you to turn off "roaming signatures" feature. You can then either launch a script on each device or deploy with, for example, a GPO if you still have local AD.
- Use PowerShell to add HTML email signatures.
- Send a configured email signature to everyone with instructions on how to add it to Outlook & customize it.
None of the above works perfectly, but neither does the native mail-flow email signature. An alternative is 3rd party email signature management tools which support Outlook email signatures. They come with a cost, but have features you can't find natively.