There are two types of sand mounds, gravity and pressure.  Both types have a pump to raise the water to the level of the top of the mound.  Once it is there, the water is either fed by gravity into the laterals or it continues under pressure.

If the sand mound is a gravity fed system, it probably has a distribution box and that is the best place to add the Septic-Scrub.  

If it is a pressure system, the Septic-Scrub can be added into the pump tank.  Add a little the first time to clean out the pump tank.  Then add the Septic-Scrub directly into the tank.  It is best to manually operate the pump to maximize the amount of Septic-Scrub which is pumped over and mimimize the amount of water.  You reach the law of dimishing returns quickly using this method.  Everytime you leave some of the Septic-Scrub in the tank.  You can never get all of the Septic-Scrub over into the field.  

If you can ensure good distribution of the Septic-Scrub through the blow out line plugs, I see no reason not the use them.  The important thing is to ensure that the proudct is distributed as well as possible through out the drainfield.

Septic Seep is recommended for use in clay soils.  Since a sand mound should not be a clay type of soil, I would suggest checking with the manufacturer regarding recommendations.  If you use Septic Seep and Septic-Scrub, Septic-Scrub must be used first because it will react with Septic-Seep.