No.

Here’s what happens if you try:

mysql> grant replication slave on *.* to 'ec2-slave'@'%';
ERROR 1045 (28000): Access denied for user 'rds_root'@'%' (using password: YES)
mysql> update mysql.user set Repl_slave_priv='Y' WHERE user='rds_root' AND host='%';
ERROR 1054 (42S22): Unknown column 'ERROR (RDS): REPLICA SLAVE PRIVILEGE CANNOT BE GRANTED OR MAINTAINED' in 'field list'
mysql>

Note: this is for MySQL 5.5, which is unfortunately what I’m currently stuck with.