Just an update to Dennis' comment from the AWS thread, this is now possible. Quote from the thread:

We just launched RDS PostgreSQL 10.5 which supports libprotobuf-c. https://aws.amazon.com/about-aws/whats-new/2018/10/rds-postgresql-supports-minor-versions/

Just an update to Dennis' comment from the AWS thread, this is now possible. Quote from the thread:

We just launched RDS PostgreSQL 10.5 which supports libprotobuf-c. https://aws.amazon.com/about-aws/whats-new/2018/10/rds-postgresql-supports-minor-versions/

The official release notes (with emphasis added):

[...]
With this update, we have also added support for extensions pglogical version 2.2.0 and pg_similarity version 1.0 in PostgreSQL 10.5 and 9.6.10; pageinspect version 1.6 extension is supported with PostgreSQL 10.5; Map Box Vector Tiles in PostGIS with libprotobuf-c is supported with PostgreSQL 10.5; wal2json has been updated to 01c5c1ec in PostgreSQL 10.5 and 9.6.10; and pg_hint_plan has been updated to 1.3.1 and 1.2.3.
[...]

^{Reference: Amazon RDS for PostgreSQL Supports New Minor Versions 10.5, 9.6.10, 9.5.14, 9.4.19, and 9.3.24}