Unfortunately I have had to maintain an LCL version for my Mac OSX customers.
fpGUI apps can be compiled for OSX - it uses the same X11 support as it does for Linux and FreeBSD. The X11 support come standard with OSX installs for many OS versions already. You might just want to create a prettier theme for OSX users - they are always a bit fussy.
I can't guarantee that X11 is always there. I think on newer Macs it's not there by default, so it's not idiot proof. I need idiot proof.
Number one on my wish list would be a third addition to the GDI / X11 pair, and it would be something that would work on every Mac OSX including the old Power PC ones if such a thing is possible.
I would want it to look exactly the same as it does with GDI / X11. If programmers want a different look for Macs, well they can program it.
I don't know much about Macs but maybe it could bypass Carbon / Cocoa and talk straight to Quartz 2D? Would that work on a PowerPC?