Another Workround:

I had a very similar problem, trying to set DataValidation to Chip from script so as to allow multiple selections from a dropdown list populated from script, which as people say is not (yet?) allowed. However you can set the DataValidation to 'chip' by hand in the sheet you are using and then set the list to 'Drop-down(from a range)' and then in your script update the data in the range pointed to rather tan the actual cell. In my case I just had another sheet with the actual data in it locked to users. Drop-down(from a range) only shows values from cells with data in so you can set the range to be the maximum that you will need.