I have not seen any policy that alter that BGP attribute the way you want it to work. I can imagine that would completely break BGP. The AS path is one attribute that must the sent and it is the complete as path that allows BGP interAS routing. You can remove the the private AS from the path, but I am not so sure you can remove the real AS paths. Juniper likes to give you additional power as the admin, so we will see what the others have to say. However. I see where you can expand or prepend ASN using a policy.
The as-overide is used fro something completely different. See attached BGP policy guide.
"Enabling the AS override feature allows routes originating from an AS to be accepted by a router residing in the same AS. Without AS override enabled, the routing device refuses the route advertisement once the AS path shows that the route originated from its own AS. This is done by default to prevent route loops. The as-override statement overrides this default behavior.
Note that enabling the AS override feature may result in routing loops. Use this feature only for specific applications that require this type of behavior, and in situations with strict network control"
A side question, what is purpose of wanting to just see a reduced number of AS path? In other words what is your objective in requesting such a potentially dangerous feature? I ask because if you want to route traffic and avoid a particular AS or to select a more preferred path, you may be able to use other features.
Can you imagine if ISP were able accomplish such feat!
[KUDOS PLEASE! If you think I earned it! If this solution worked for you please flag my post as an "Accepted Solution" so others can benefit..]