This sounds cool.  I'm sure it'll ease the transition from the TI-OS to KOS.   Good luck.

This sounds awesome, I remember hearing about porting ASM programs to KOS once it's done but this would make it much easier. Good luck on this

so what all types of functions would be realistically feasible for being run under this? would all bcalls work? and i heard some people asking about the VAT

The VAT will be a fake VAT set up with any .8x* files from the current directory.  Any changes to it will be reflected upon exit.
For the record, multitasking will be disabled, and I'll try to set up the RAM as best I can to mirror TIOS.
Compatible routines will probably be everything relating to text, and drawing.  Ion/MirageOS support will be there, and perhaps DCS7 if I can make the KOS GUI system look similar.  Most VAT editing things will probably be mirrored.  Hooks will not be supported, but it will not crash when you try to make one.  Ans will be faked.  Depending on how I handle it in KOS itself, floating point support will be considered.  Any routines from KOS that are similar to any routine from TIOS will be supported.  Any bcall that is unsupported will instead point to a routine that quietly returns.  Flash writing will not be enabled, and will in fact be completely sealed off.  Flash will be locked, and if at any time during the application's execution the boot page is swapped in, it immediately swaps in something different via the interrupt.  I'll probably check the program to see if it wants to write flash and tell the user it would be dumb to run it before executing it.
I will also make a fake bcall that can be used to determine if your program is running under Compat8x or not.

I plan to natively support Phoenix however, I see no reason why it couldn't run...