I don't think that this has been supported for a very long time... Also, installing anything but VIOS from the HMC was quietly removed from the code there[1]. So for an AIX network installation, you choices are NIM or a PowerVC image-type installation.
For Linux, you could build the installation bits by hand on whatever server you wanted (copying them from Linux, it just needs BOOTP/DHCP and the images to be in the correct place on a TFTP server).
IMHO, managing Linux from NIM was a boondoggle anyway. They should just give us packages that set-up the installation area (in both directions). NIMoL was not a bad idea, but they shouldn't have tried so hard. Just getting through the installation is really enough, we have other tools to maintain the systems. The results were not worth the effort, but that was because they picked the wrong amount of effort.
[1] Both of these burned me significantly on the same project and I'm still bitter. I yelled a lot, but no one cared. Well, some people cared, but nobody could solve the problem.
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Stephen