yes, it is possible to host lobbies on 9199. Currently the easiest way to do it is using freeboot, because you no longer need 2 nands. To do this though, you need the xbox's original nand dump & CPU key. Once you have both of those, you can use this tool to create you new freeboot 9199 nand :
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Just enter your cpu key in designated box, then click generate. It will ask you to open your original nand, locate it and click open. Then dump the nand using flash360 if you have xbr insalled, or dump with LPT if you dont.
Once you have done this, you can then copy mw2 to hard drive using xexmenu, and copy accross the modded files.
Here are some suggested modded files for the mw2 directory :
Patched Default_mp.xex and Patch.ff :
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Code Post with fast restart :
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You also need to add the files for tu5, but i cant find them atm. ill have a look around and link to them later if i can find them
Hope i helped!