It looks like that hack is also having issues with Sprite Zero...
Older NES games relied upon something called "sprite zero hit detection" in order to create static status bars which remained in place while the game level itself scrolled around. Basically, a sprite was positioned within the status bar (sometimes underneath an identical background tile, as in the case of SMB1), and the Nintendo would be alerted when the scanline being drawn intercepted a non-transparent pixel of Sprite Zero which over/underlapped a non-transparent pixel of background tile. This signaled the beginning or end of the status bar (depending upon whether it was located at the top or bottom of the screen).
If that sounds precarious, well... it kinda is. If that sprite or background tile is altered in a way that significantly delays or prematurely triggers the signal, you're going to wind up with split-scrolling gibberish. Or, in the case of those bad dump SMB1 ROMs, a game that crashes/resets as soon as the screen tries to scroll anywhere.
The Good News: What you seek is entirely doable! (PM me for details.)