Turns out the complexities of the Apple II artifacting introduce subtleties that aren't immediately apparent when you're trying to reproduce it on a 4-colour display. I first noticed this when I got to the stage where my 2BPP rendering routines should have produced a faithful static display - yet I had gaps between my brick tiles.
After chasing red-herrings for a while it occurred to me that the tile data I had imported wasn't quite right - the bricks, for example, were only 9 pixels wide, not 10. So I went back to the MESS algorithm in my import routine, and deduced that the root cause was the fact that the artifacting is dependent on the bytes following the pixel in question! Because I was rendering single tiles in isolation, the artifacting algorithm wasn't producing the 10th blue pixel on my bricks.
I subsequently modified my import routine to render two (2) of each tile next to one-another, and lo-and-behold the gap disappeared! After (eventually) realising that I had to wipe the 2nd tile before importing the tile data, I finally had no gaps in the bricks and the graphics all looked, as far as I could tell, correct.
On this point, it is actually impossible to 100% accurately render the artifacting, because bricks adjacent to gaps or ladders or ropes are actually rendered only 9 pixels wide on the Apple II, whilst adjacent bricks are 10 pixels wide. It's also somewhat evident via the gaps next to the ladders. However, the effects are so subtle that they're practically undetectable, and they certainly have no bearing on game-play what-so-ever.
Similarly, there's some fringing effects on the guard and player sprites; however for the most part they are identical to what gets produced on the Apple II (according to MESS at least), except for complex interactions with multiple guards and backgrounds etc, and even then, they're subtle and again, have no bearing on game-play.
Overall, I'm very happy with the colour version and it certainly captures the look and feel of the Apple II version. And after several months of play-testing the monochrome version which, incidentally is the version I'm traditionally familiar with, it does look quite a bit nicer!
I'll do a bit more testing, and I want to attempt to centre the screen and also add the high score initials entry (tedious but trivial) before I release the next beta demo. That will probably be the last beta before the full release, which will include sound.