The index is stored in the plugin's metadata location, i.e.
[workspace root]/.metadata/.plugins/org.maven.ide.eclipse/nexus
There will be one folder for each remote repository index in use.
You can configure the plugin to not download the index at startup too. Got to Window->Preferences->Maven and uncheck Download repository indexes at startup, you'll have to remember to reactivate it to get any updates though
Update:
I just verified that copying the metadata works. M2Eclipse will still contact the repository to download the deltas (assuming the above option is checked), but that only takes a few moments as it is only downloading the deltas.
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