Does anyone know if it is possible to sort a Suitelet Sublist by a sublist value? I can click the header label to sort in the UI, but would like the sublist sorted when the page is written. Can I sort the populated sublist via SuiteScript or do I need to setSublistValue() in the sorted order?

It will default to the order you add the rows in SuiteScript. So you just need to sort ahead of time.