Can I use the location name as the filter instead of the internal ID? If so, how? Do I need to use Builtin.DF?

yeah this seems to work for me

select
  location,
  builtin.DF(location)
from
  transactionLine
WHERE
  BUILTIN.DF(location) = 'xxx'

you could also join directly join onto the location table

select
  location,
  builtin.DF(location),
  location.name
from
  transactionLine
  inner join location on transactionline.location = location.id
  WHERE
  location.name = 'xxx'

Thank you @Matt Bernstein