No feedback was received, so I came up with a job to solve my problem. This is not an elegant solution, but it works.
First, I combined the Assignment and Billing Objects into one Master object so that all the information is contained in one object called Assignment.
Secondly, I changed the GetAssignments method parameter to take a string, not a list of integers. And then from Crystal Reports, when the user is prompted to enter a value, the user can enter a comma in a separate list of destination identifiers. The web service breaks the Assignmend identifiers and returns List of Assignment objects.
The "Allow multiple values" option in the "Edit Parameters" window in the Crystal Report Viewer is unavailable / not supported / always false when using web services as a data source.
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