I can report a SUCCESS
The 29150 chips came today and were a tad larger than I imagined. Unfortunately, they came with shorter pins than depicted on the website so that left me with fewer mounting options as you will see.
First photo shows the current 3.3v 1117 chip in situ (I had previously shown the position of the 1.8v chip in error), and the 29150 chip placed on its left. Given the proximity of the capacitor and the little wiggle room left by the 29150's stumpy legs, I decided to eschew elegance in favour of attaching leads to all the legs and finding somewhere on the PCB to attach the chip later.
The second pic shows the finished solderwork. No prizes for workmanship but I decided to do the least I could so I could reverse it if this left me with a non-functioning card.
The third pic shows the card that I placed in my DA, which previously would not even complete POST with any 3112 card in it, flashed or not. SysProf shows it as a SeriTek 1S2, if the image isn't clear. I also checked it in a Sawtooth under Tiger to confirm it wasn't a fluke. Looked good to me.
Sadly the second 3112 card I got is a dud. Won't complete POST in any PC or Mac I tried it in so I only have this attempt to show you. All in all, the MX29LV040 chip cost me about £1 each in batches of 5 and the MIC29150 £3 in batches of 2 delivered, so it isn't expensive even if it goes wrong.