mr 3.6+ matches Maya 2008 SP1/Ext2, so Maya 7 is a considerable number of versions back. Remember that the maya shaders have to match, too, and with that amount of difference, I expect there would have been considerable change in them.
In theory, if you could compile the Maya 7 shaders for 3.6+, it might work, but then the translation export to mi files might depend on some of the changes as well. It is possible, that you might be able to use one Maya 2008 SP1/Ext2 license to convert from the old maya scene to a new one, and then export it to be compatible with mr 3.6+. I'm not sure how many versions back each Maya release supports regarding its scene files.
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