I believe you could do this with two parameters and one calculated field as a filter.
Parameter1 as a string (permit any value)
Parameter2 as a string (permit any value)
IF [Parameter1] <= [TextField] AND [Parameter2] >= [TextField]
Filter the Calculated field on 'Show' and whatever you enter in Parameter1 will be the "alphabetical start point" and Parameter2 will be the "alphabetical end point."
Thanks a lot Mark. I was able to implement your idea and it worked.