I think your take would also require bitmap mode based on the colour combinations used, which would complicate scrolling significantly requiring AGSP scrolling. While the versions posted on Lemon64 are all character based.

Interesting! I never saw that thread in my life, as I don't frequent any C64 forums, nor even played a single C64 game in my life, entire idea of it came from a piece by Vierbit where he made a Metroid C64 "port" - http://pixeljoint.com/pixelart/37247.htm - I simply thought the widepixel aesthetic looked cool and decided to give it a go myself.

The version you posted is definitely more accurate to the original game. If my version was made into an actual playable thing, it'd need to be heavily rebalanced compared to NES original, and it'd probably require serious level design changes to accomodate different hitboxes and jump arcs - but the Lemon64 one could easily keep most of the original design intact. However, I imagine that using single sprites for Batman and enemies could allow my version to get away with more enemies and projectiles on-screen.

This is remarkably similar to work that was posted on the lemon64 forums here: 

https://www.lemon64.com/forum/viewtopic.php?t=71776&start=30

and has also appeared in parts on Facebook.

It appears that you are not the only person who thought of developing this. Your take using a single sprite size to represent batman is somehwta different through.