I now have a working cross-compiler and a C runtime environment and libc based on gcc 4.9.2 and newlib 1.18.0 with floating point emulation. I have created a linker script for the T030 along with some startup code and the first prototype of my own C based ROM monitor is now ready:

T030 ROM BIOS v1.00                                                                               
-------------------                                                                               
                                                                                                  
Checking RAM: 512 KB OK                                                                           
                                                                                                  
#> h                                                                                              
d - Dump address: d                                                                         
h - Help                                                                                          
g - Run: g                                                                                  
r - Dump registers                                                                                
u - Run in user mode: u                                                                     
w - Write single byte: w                                                              
x - Transfer file with XMODEM: x                                                            
                                                                                                  
#> x 90000
Starting XMODEM receive...                                                                        
CCC

Done, bytes received: 36992, write errors: 0
#> g 90000

T030 TEST MONITOR v0.02
-----------------------

*> r
SR: 2100
SSP: 000fffa0
USP: fffffffe
VBR: 00080000
CACR: 00000000

*> g 000000
Exception #4 (illegal instruction) SR: 2104 PC: 00000000

A simple interrupt routine to use one of the MFP timers to blink a led can be written like this:

void __attribute__ ((interrupt)) timerb_isr()
{
	static uint8_t c;

	if (c++ >= 50)
	{
		if (bit_is_clear(MFPGPDR, 0))
			bit_set(MFPGPDR, 0);
		else
			bit_clear(MFPGPDR, 0);
		c = 0;
	}
}

void enable_led()
{
	MFPTBCR = 0;
	MFPTBDR = 255;

	// enable interrupt for timer B
	bit_set(MFPIERA, 0);
	bit_set(MFPIMRA, 0);

	set_vector(72, timerb_isr);

	// start timer B
	MFPTBCR = 7;
}

I have also updated the glue logic in the CPLD so the CPU now runs at the full speed, a whopping 25 Mhz.