You could get a fair estimate by opening the download page for your plugin then change the plugin number in the URL up or down one digit and strip off the plugin name (the number is sufficient) and see what the dates on those files are. Since the IDs are assigned sequentially, the ones immediately prior or after yours will probably have been uploaded within a couple of days, in some cases minutes, of your original upload. For example, for CMapins, the URL is http://www.lotrointerface.com/downlo...3-CMapins.html
and stripping off the name and decrementing the count yields Titan Bar's download page http://www.lotrointerface.com/downloads/info692
Of course, this only helps if the preceding or following authors did not remove any of their early versions. I don't think the creation date is published anywhere else.