With PostgreSQL on Amazon RDS your choice of extensions is limited to the extensions they explicitly support. As far as I'm aware there's no way around this limitation.
Like you mentioned, the general option in this case would be to host PostgreSQL yourself on EC2 instead of RDS. You lose automatic backup/replication/management features, but you get the power and flexibility you need. This will certainly work but will require some leg work to replace what you're losing by not using RDS.
The only alternative to this I can think of is that you may be able to host a different (otherwise empty) PostgreSQL server with the oracle-fdw extension installed and use the postgres-fdw extension (which is supported by RDS) to proxy requests from your RDS hosted database, through your proxy PostgreSQL database, to your Oracle database and back. If the amount of data you're retrieving is substantial, or if the number of queries per minute is high this is probably a terrible idea. But it might be worth testing to see if it works for your use case.
I did a quick search around and I haven't been able to find any references to anyone actually layering foreign data wrappers like this but I also couldn't find anything in the manual or online saying it wasn't supported either. In theory it should work, but if you do try it make sure you thoroughly test it prior to using it to do anything important.
