Hi Jack,
I would like to follow up with some questions :
About Mysql support you're right. The databases supported are Oracle, SqlServer and DB2.
For the RTC 4.0 DB versions and compatibility you can check at this link : https://jazz.net/library/article/811
On the question about querying a remote database, An alternative to approach it, is to create this connection using only the databases. Do you connect to the RTC database the remote database that you want to query. I believe it's a good solution to this case because it is transparent for the application.
For Oracle databases, we could use the DBLink (database link) which is quite simple to enable :
Check1 ) http://www.orafaq.com/wiki/Database_link
Check2 ) http://docs.oracle.com/cd/B19306_01/server.102/b14200/statements_5005.htm
For DB2 you can create synonyms referencing the external Data : http://forums.devshed.com/db2-development-114/equivalent-database-link-oracle-db2-77675.html
For MySql Is had research and I could find some orientations regarding the federated engine should be use for query tables on remote servers.
I hope it help you out.