If you do an advanced search and search for a specific user you can specify from a range of dates for which to search by. Not ideal but the best I could think off.
You can find a member's first (i.e., oldest) 1000 posts by using Advanced Search and setting Sort Results by to "Last Posting Date" and "in Ascending Order".
If you really want to get specific, you can Search in particular forums, one or a few at a time, and see a user's first or last 1000 posts in that forum.