Coming from underneath is the only way I've ever been able to get to the block plug and even then it's a pain in the butt.

BTW, did you think to de-oil prior to doing the citrus flush. The MB service manual procedure has this as the first step and specifies some MB de-oiler, but 1/2 cap of tide or the like will work fine. Flush with good clean water 2-3 times and on with the flush.

Also, 1 pound of citric acid is sufficient for a good flush. Finally, I use purified water with the coolant...

-Ken