I would avoid the replicate-do-db option due to the restriction on cross-database replication and users not fully qualifying a database name or not specifying "use dbname;" (it will execute on master but not replicate to slave).

Use this option instead (put it in my.cnf):

replicate-wild-do-table=dbname1.%

replicate-wild-do-table=dbname2.%

Or you can specify the tables, replace % with table name, one on each line. http://dev.mysql.com/doc/refman/5.0/en/replication-options-slave.html#option_mysqld_replicate-wild-do-table

This link should help get you started: http://dev.mysql.com/doc/refman/5.0/en/replication-solutions-partitioning.html

You can safely write to a schema on the slave if that schema does not exist on the master or if you correctly use replication filters to prevent its replication to the slave.

"Correctly" is the important word in that sentence. Refer to this page:

https://dev.mysql.com/doc/refman/5.7/en/replication-options-slave.html

Check the --replicate-do-db and other filter settings, and note that the behavior changes based on whether binlog events are formatted as rows or as statements. When statement-based replication is being used, replication filters only apply to the database in use as the current database. When row-based replication is used, the filters are applied regardless of current database.

Be sure you get this right, otherwise, you could have a situation like this:

replicate-ignore-db=monkey
binlog_format=STATEMENT

Then you do this:

USE zebra;
TRUNCATE monkey.tablex;

Guess what? tablex just got truncated on both master and slave. However, if you had this instead:

replicate-ignore-db=monkey
binlog_format=ROW

and did the same TRUNCATE on the master, monkey.tablex would not be truncated on the slave.

Note that this all applies according the binlog_format in use at the time the event is logged, and since binlog_format can be changed on a per-session basis, you can get some surprises unless you know exactly what's going on.

Having said all that, if the schema you're writing to does not even exist on the master, then you have nothing to worry about, other than the fact that you can't have read_only or super_read_only set on the slave if you plan on writing to it, so you can't count on it preventing write activity.