Counter weight for added back storage area/deck

Sounds like shade-tree engineering to me. Creative, but could be a problem with exceeding your trailer's max weight.

600lbs would be 10% of my total TT weight (packed for camping) and it would eat up a large chunk of my cargo capacity for the whole trailer.

I'd suggest you get your trailer weighed as best case, doing a tongue weighing at minimum. You want and need positive weight on the tongue for handling reasons.