Although you won't be able to disable email validation (Zimbra only likes valid email addresses), you may be able to workaround the issue from the Zimbra side. On one of your Zimbra mailstores, create the domain "local" and route its mail to the Exchange MTA. Then create an alias domain that is valid that can be used in the Zimbra admin console. Issue all commands as the zimbra user.
Create the "local" domain:
Create the alias domain "local.forward" (local.foward is valid in the eyes of Zimbra):
zmprov cd local zimbraMailTransport smtp:exchange.server.tld zimbraMailCatchAllForwardingAddress @local zimbraMailCatchAllAddress @local
Modify Zimbra's MTA configuration to not append it's domain or hostname to non-fully qualified addresses:
zmprov cad local.forward local zimbraMailCatchAllForwardingAddress @local
You should be able to specify whatever address you want in the forward in the form of "email@example.com". The address will be translated to "user@local" and routed according to its zimbraMailTransport value.
postconf append_at_myorigin=no append_dot_mydomain=no
I hope this helps.