You may consider TUXRACGROUPS. Environment variable TUXRACGROUPS allows user to define a "super group" (RAC group) which contains one or multiple Tuxedo groups. All tpcalls within a global transaction will be routed to the same "super group". This feature was designed to Oracle RAC first so the environment variable name contains "RAC", but this feature can be used in other scenario which requires global transaction & Tuxedo groups affinity. In your case you may configure the GWTDOMAIN and GWSNAX running on the same machine in the same RAC group, then You can refer to http://docs.oracle.com/cd/E35855_01/tuxedo/docs12c/ads/adorac.html#wp1058798 for further information.
This solution has been adopted in a similar customer scenario a couple of years ago.
I don't think TUXRACGROUPS will have any impact on requests that aren't part of a transaction. Another possibility might be to try client/server affinity. I'm not certain it works from WTC across domain gateways, but it would be worth a try. If it doesn't work, I think an enhancement request makes sense as it would be reasonable to have WTC support client/server affinity.
Oracle Tuxedo Chief Architect