Hi,
I use (A)T89C51CC01. There is no problem to write program flash if you are familiar with programming and hardware. First of all, you should read datasheets about boot-loader, its user API, In-System Programming and Flash Library. Search Atmel site for it. I cannot remember from where I obtain all these docs - if you need them then I can send it to you via e-mail.
In short, built-in boot-loader has the user API for a lot of functions you may call from it. For example, to write program flash I use something like (sorry if a typpo somewhere - my routines is a bit different but hardly to understend then):

; ...
	MOV	AUXR1,#0		; DPTR0
	MOV	DPTR,#DEST_FLASH_ADDR	; target flash address
	MOV	AUXR1,#1		; DPTR1
	MOV	DPTR,#SRC_XRAM_ADDR	; source address in XRAM
	MOV	A,#NUMBER_OF_BYTES	; size
	MOV	R1,#9			; API command: write code page
	MOV	AUXR,#00001101b		; attach internal XRAM
	CALL	SUB_API_BOOT_FLASH	; move bytes from XRAM to FLASH
; ...

; subroutine to call user API
SUB_API_BOOT_FLASH:
	PUSH	AR0			; these regs may be affected
	PUSH	AR2			; with boot-loader routines
	CLR	EA
#ifdef _WATCHDOG_USED_
	MOV	WDTRST,#0x1E		; restart watchdog if used
	MOV	WDTRST,#0xE1
#endif
	ORL	AUXR1,#00100000b	; map boot-flash into F800...FFFF
	LCALL 	0xFFF0			; address of API_ENTRY_POINT
	MOV	FCON,#0			; reset flash control register
	ANL	AUXR1,#11011111b	; unmap boot-flash
#ifdef _WATCHDOG_USED_
	MOV	WDTRST,#0x1E		; restart watchdog if used
	MOV	WDTRST,#0xE1
#endif
	SETB	EA
	POP	AR2
	POP	AR0
	RET

Good days!