Does anyone do item fulfillment out of NS without a WMS?

I have many customers that don't use a WMS and fulfil out of NetSuite.

We do. I sense there is a question here...

trying to figure out how to "mass fulfill" when there are serial numbers involved

Suitelet is the best way 🙂 but you have to be creative 🙂

We don't have serial numbers, but lot-numbered inventory. I had to develop a script + custom record to create labels for each of the cartons of our lot numbered goods. I guess the basic idea here is that our inventory is already lot numbered and say each case pack is 40, once a fulfillment is created - the lots are already picked from inventory. Then, the script creates a custom record for a label to be applied on each of these cartons. If the serial numbers are not already created, you would likely want to create records for the required number of serialized inventory records, then download that data to print out labels / gift cards for these serials created by script.