I don't see a huge issue. The nim server doesn't need to be on the same physical subnet as the server it is performing maintenance on. Obviously you'll have potential firewall issues that you may or may not have to work out.
I think the biggest potential issue would be network throughput (not necessarily latency). Do you have a good connection to your nim server? Assuming you'll be dumping mksysb images to it, you'll need to have a good amount of bandwidth or they'll take a long, long time. Same goes for patching and other maintenance tasks.