The Law Co. from Jakarta that advertises at the top of this page offers a good explanation.
Postnuptial Agreements in Indonesia
In summary, a pre-nuptial agreement is well understood in the 1974 marriage laws of Indonesia but the post-nuptial isn't even mentioned. One has to go back to some constitutional civil law to find reference.
The pre-nup is often used by mixed marriage couples to permit the WNI spouse to own freehold land...and is the principal reason, in RI, to have that notarised.
Post-nups are agreements that can be notarised, but do not seem to have the maturity of Indonesian law to be fully comprehended in terms of mixed marriage prevention of foreigners from certain RI regulations, in respect to property.
I've never heard of a post-nup being used to allow the WNI spouse of a mixed marriage to own freehold land. It's an interesting point, however, and I'd love to see the success of this creative idea.