Warning: I know very little about DB2 on Z/os so some of these questions are basic, bear with me.
There is an existing extension for DB2 on iSeries at https://github.com/liquibase/liquibase-db2i. That is different than your z/os support, correct? The z/os support should be moved to an extension rather than directly off the liquibase code, that way it can more easily be updated and managed independently of liquibase releases. I can help with moving it into an extension is you would like.
You have both a DB2 LUW and a DB2ZOS Database object. What are the differences between them and what are the two types for?
Nathan