If you don’t want to hack anything irreparably you could try using a 26-pin stacking header on the Pi to extend the pins above the obstacles and just plug the 40-pin pHAT on top.
Couldn’t find anything in Pimoroni’s listings but the Pi-Hut have an adafruit header you could use.
There is another adapter you might possibly consider from AB Electronics - they make a Pi3-40 pin to Pi2-26 pin adapter board. Actually intended to allow old 26-pin HATs to be used on the newer 40-pin Pi’s. It is a passive board (no converter chips). By swapping the header genders it might be possible to provide reverse conversion if you can live with the mechanical arrangement. You’ll still have missing pin function beyond pin 26 but that can’t be helped.
Hmm, I was mistaken in my assertion that “the pins that are needed are probably in the first 26” - the pHAT DAC needs I2S pins BCM19 and BCM21 which are on 35 and 40 of the later connector, and absent from the P1 on my old model 1B. Which is why my pHAT DAC doesn’t work!
So the question is - please can anyone advise where (if?) these can be found anywhere on an early model 1B. I don’t have a P5 connector!