So, I didn't give up. I've now compiled universal binaries with SDL
Framework and EmuTOS - still not convinced whether it does pick up a
users install of SDL (See Anders' comment) or if it's just a problem if
a user has a mismatch of i386 libs while running an x86 app.
However, while waiting for confirmation I started to take a look at
making sure I had libpng, portaudio and capsimage support in the build
as well - and here I did run into issues:
The capsimage available for download at http://www.softpres.org/download
(developer distribution) does contain a Framework and an include file.
But only a single include file (/caps/capsimage.h). Hatari tries to use
another one, /caps/fdc.h - which I did find and copied from fs-uae
github. However, there's obviously a version mismatch now - "use of
undeclared identifier DI_LOCK_DENALT" when compiling floppy_ipf.c).