If MaxScale is routing a write to an Xpand table (and only an Xpand table), there should be an options to bypass Server and write directly to the Xpand backend. This assumes there are server definitions for both MariaDB and backend Xpand instances.
Perhaps it can be thought of as a service comprised of two services. While the MariaDB service may use a read/write splitter to funnel writes to the primary, the Xpand service can load balance writes across all backend Xpand instances.
The top level services simply evaluates the query to determine whether or not it can route a write to the Xpand services. If not, it routes it to the MariaDB services.
This assumes MaxScale can get tables definitions and maintain metadata about the storage engine used by all tables.
The goal is to optimized write performance (throughput and latency) when writing data to Xpand tables (and only to Xpand tables).