There are options to sort the members list by either Username, location, post count, joined date or website. You can also sort these either ascending or descending.
i.e. if you are searching for usenames beginning with "U", why not sort by "Username" and "Descending"? If you get really clever, you'll have a rough idea as to the number of page jumps you need to make.
Finally, if you are
really clever, you can edit the url of the appropriately 'sorted' list with the knowledge that each page contains 25 usernames... simply modify the number at the end of the url to reflect the number you wish to start from e.g
http://www.rumration.co.uk/cpgn2/Members_List/mode=joindate/order=ASC/start=50.html
would list usernames from 51 to 75...
Change the number to 250, and find usernames 251-275.... (approx "bil" to "biz" if sorted ascending by username!)
Hope this helps!
Ghostie