Hi Tom,
I think you could probably try to define a workflow using SharePoint Designer which when instantiated will populate the random number field. If there is no out-of-the-box random function available in SharePoint Designer, if could be difficult to accomplish what you're trying to do without using custom code. Even if you are using GUIDs to build new values, you would have to extract the numbers out of the GUID values which have alphanumeric characters. There is also no guarantee that the extracted numbers are unique IDs so you could end up with duplicate values.
I think the most straight-foward way is to write some custom code to create a new workflow using Visual Studio. You can then use the Random class from Microsoft to generate a random number and update the number field in a workflow activity.
Yuan