Unfortunately it’s not currently possible nor does there appear to be will to consider it as a modification as you can see by this recent thread on the wp-hackers list and this ticket on trac.
If you’d really like to see this be revisited I’d suggest:
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Present your case on wp-hackers but be forewarned your use-case better be good and not “security through obscurity” or it will get shot down as above.
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Present your argument in a trac ticket with the same caveats.
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Even better, upload a patch to trac that enables your desired functionality. It’s much harder to say no when the work has already been done (but of course, they do have a preference for saying “no” a lot more often than they say “yes” so be forewarned.)