Thank you for sharing this, definitely looks like the blksize isn't variable here. Next would be to see if AIX open boot supports the http interface for receiving the boot image.
I will try modifying the /etc/bootptab to define the tftp filename as a URL and see if that works. Let me know if at the surface if the below bootp config should cause Open Firmware to retrieve the file over http, if IBM Open Firmware supports http...
eg. (using data from here
Hope that works tomorrow, otherwise time to build out a new NIM service.