I wonder how the performance of your partial index would compare to a pgsql materialized view implementation? [0] Did you by chance explore such an option?
One stuff to keep in mind is that Partial index, index are internal implementation of the database. If you start using materialized view it is visible to the application.
I think you are adding another entity that you must now support and that the application must specifically call.
If your application supports multiple databases, i don't think you want to have specific code to support optimization. Of course you need to do this :)
Materialized views are inherent to oracle. They might refer to them as snapshots. In mssql they are known as indexed views. There is no implementation in pgsql but it is often discussed [0]. I gave up on data portability for nontrivial applications a long time ago. If you mean instead a view across multiple databases, it might be possible using pgsql foreign data wrappers but definitely not performant.
[0]: http://www.pgcon.org/2008/schedule/attachments/63_BSDCan2008...