This afternoon, I noticed a tweet from Richard GW1JFV saying that he'd worked K1GUP on 29MHz AM and went on to say that he used his FT8900. I happened to be in the shack at the time and had the instruction manual to hand.
Menu item #45 was quickly found AM on | off - which allowed me to toggle AM on to 29MHz.
I fired it up and it sounds ok. Not brilliant on the monitor receiver - perhaps a little harsh, but it's AM nevertheless.
Another option for using AM on 29MHz. Thanks Richard!
Update: Oh dear - this turns out to be wrong! The 8900 is receive only on AM. After an interesting conversation with Chris G3VEH, I checked this. Although the 8900 transmits when AM is selected, if you listen, it's actually FM. When Richard made his QSO, his QSO partner must have slope detected his FM!