Read you have your second MX5, nice! I was thinking of doing exactly the same thing, but have decided to hang on to the RX8 for now. Did you disable the passenger airbag? From the research I did on it, having a proper child harness and sitting the seat right back was more than good enough.
Nope, didn't do anything with the airbag. Alex is nearly 8yo now and has been out of the child seat for ages. But even with a booster seat and lap-sash belt, there's no problem having younger kids in the passenger seat. It's only with baby capsules and proper child seats that airbags are an issue.
Standard 2006 NC - YouTube WP 1:11.89 | SMP-S 1:05.90 GP 1:54.93 N 1:18.09 L 2:22.49 | PW 1:02.52 PI 2:00.55 | W-S 1:12.44 W-L 1:43.36 | SR 1:33.25