OOH, You are just the person I need!
The actual mounting part: Can you unscrew the ring around that balljoint to put in a different ball joint?
I want to use this instead of something that attaches to the L-hook bracket thing.
None of their compatible phone holders that attach to the actual mounting bracket appeal to me.
Not sure what mount they use in NZ, but if you are looking to mount a smartphone to a foot-stretcher, look at the modular mounts used on motorcycles and bikes. Two challenges for rowing iphone mounts are: to be quite secure to avoid dropping the phone in the water and to enable adjustment to any angle to make the screen readable in a wide range of light intensities and directions. Here's what I found worked for my 1x (though there are more options now than 4 years ago when I ordered these parts).
You can attach this widget with a bolt to your foot-stretcher to provide a base for connector and phone holder:
https://www.amazon.com/gp/product/B0012TRIY4/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
...then use this connector which rotates in all directions:
https://www.amazon.com/Mount-Double-Socket-1-Inch-RAM-B-201U/dp/B000UGZTQO?ref_=ast_sto_dp&th=1
...and this widget to hold your phone (also moves in all directions on a ball joint):
https://www.amazon.com/gp/product/B0055PH0XA/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
