As far as JTAG is concerned, the Westell router reports the CPU as a 6358, which tjtag does support.
HOWEVER, the Westell has 2 devices (parts) in the JTAG chain: there's also a 6318 which is the wireless chip. Unless it has been recently fixed, the native tjtag does not support this; it is expecting only one device in the chain.
az1324 created a modified tjtag that bypasses the 6318, and others have been successful using it. You can get it here:
http://www.mediafire.com/?mywmluzmulw
If you're using TUMPA as your JTAG cable, neither the original tjtag or the modified one can connect to it.
zJtag will connect to Tumpa, but can't handle the situation with the 6318.
A more generic utility that many use is UrJTag, which was designed for a more universal purpose than just eJTag. UrJTag can handle multiple parts on the jtag chain. Hoever it has limited support for ejtag as there aren't that many devices defined in it. The 4318 is unrecognized, and so it can't go much further than informing you of the IDs on the jtag chain.
etn45p4m was able to make some headway in using UrJTag by creating a definition file for the 4318, basing it on the bcm6358 file but couldn't get it to detect the flash.
By removing the EJTag references (register, instruction code, ejtag_dma, endian type), I was able to get UrJag to recognize the flash.
I had to change the instruction length in the file to 8, as well as extend the bits in the instructions to 8 bits by padding (prefixing) 3 0's to sample/preload, idcode, and 1's to bypass. However so far I can only successfully run detect once.
UrJtag unfortunately has no function to back up the flash to a file.
Someone was able to install OpenWRT to the Westell using tjtag_w7501, his instructions can be found here:
http://hanschan.no-ip.org:8080/wiki/index.php/Openwrt_on_Westell_7500
I was able to use UrJTag to install that firmware.
If you didn't already do so, you'll want to connect to the TTL serial connection on P502, particularly considering that the switch connected to the network ports on the router are initially disabled and have to be activated from the console. The network internally is working alright, but you have no physical means to get to it otherwise.