The modification that you propose is very commonly done on the Goat Island Skiff.  (In the pict, the halyard is on the right and and a 6:1 vanghaul is on the left.)  From my experience, this is an excellent mod because I can easily raise/lower the sail and controll luff tension from aft. 

You actually have three places that you must be concerned about regarding added loads.  The first is the mast step.  It will have added loads approximately 2x the halyard tension.  The mast step on my goat is 1'x1' hard wood 1" thick glued directly to the bottom so no problem. 

The second location is where the turning blocks mount beside the mast.  Those locations will each have a load approx 1.4x the halyard tension pulling upward and aft at about a 45 degree angle.  (Total load of both blocks applied to the seat is 2.8x halyard tension.) This is the vector sum of the vertical load pulling up and the horizontal load pulling aft.  When I built the Goat, I planned for this mod by making my partner a bit bigger.  These blocks are through bolted through the partner which is also 1'x1'x1" hardwood with bracing on all four sides and glued to the back of the seat top.  

The third location is where the cam cleats are installed.  They have to take a load equal to the halyard tension applied aft.  In that location, I added another 1'x1'x1" glued to the underside of the seat and the two bulkheads.  The cleats are also through bolted.

On the GIS, it is common to sail with very high luff tension in high winds.  This is why the designer recommends a 4:1 or 6:1 vanghaul.  I would guess in those conditions that the halyard tension is something 100-150#.

This is not a lot of help with your NED but it should give you some idea where you will have to beef things up.  I would suggest that CLC can give you an idea regarding strength of the step and partner.    

   I added blocks and cleats to stainless plates through bolted to the mast thwart so the lines for the halyard, downhaul and lazyjacks are easy to adjust one-handed. It has made singlehanding much easier and also works better when directing someone else. I'd be happy to add a photo but can't seem to figure how.  Tinypic won't load. Barry

   Would like to see your photos if possible.  Have you tried https://postimages.org/

 I can't find a closeup but will shoot one and add later if it would help. Left is downhaul, right of mast are halyard and lazyjack line.  Each runs through a spring loaded swivel block and a cam cleat.

  Is there any reason a modification such as this lower forward deck/mast support could not be adapted to the NE Dory lug rig?