For Redirector keys, all you have to do is follow the normal convention of using $foo.rawValue$ always for links, redirects and human-readable headers, and using the $foo$ key itself only for cases where it's being fed directly into the Splunk search language.
This will work for multiselect Pulldowns as well as single-selection Pulldowns, and indeed it will note the array-valued selection and you'll end up with values in the URL like ?Weight_Select=value1&Weight_Select=value2&Weight_Select=value4.
And in the target view, when URLLoader sees that value, assuming there's a multiselect Pulldown module with a name param of "Weight_Select", it will be able to prepopulate itself to exactly those values when the page loads.
This point, about when to use $foo.rawValue$ and when to use $foo$, is called out as being an important point on the main docs page for using Redirector and URLLoader - go to either "Key Techniques -> linking from view to view -> intro", OR go to "Module Documentation > The URLLoader and Redirector modules". Both of those links actually go to the same page. Note the third bullet item in the "Notes" section.
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