Avoiding "Waiting for table metadata lock" when `ALTER TABLE DROP PARTITION`?
What you are asking for is impossible. Regardless of storage engine, DDL of any kind will lock a table. If you must remove a partition from a table that is active, you should:

Setup MySQL Replication (if you haven't done so already)
Perform all SELECTs involving v3_cam_date against the Slave
STOP SLAVE; on the Slave
Perform ALTER TABLE ... DROP PARTITION on the Master.
Perform all SELECTs involving v3_cam_date against the Master
START SLAVE; on the Slave (Replicates ALTER TABLE ... DROP PARTITION to the Slave)

This is probably your only recourse. The only other recourse is to simply wait out the ALTER TABLE ... DROP PARTITION.
This situation requires some intervention in the application. Within your application, you would have to create a Write DBVIP on the Master and use the Read DBVIP with one of the following three(3) options:
OPTION #1

Keep the Read DBVIP on the Slave

OPTION #2

Keep the Read DBVIP on the Master
When you must do an ALTER TABLE, move the Read DBVIP to the Slave
When ALTER TABLE has been completed, move the Read DBVIP to the Master

OPTION #3

Setup Read DBVIP on a LoadBalancer
Remove Master from LoadBalanced DBVIP on demand

Option #1 seems to be the simplest way in the long run.