We always had some CAD dude write scripts in-house.
We (I always worked inside semiconductor foundries)
did not let customers do the fills.
However you might try the foundry's tape-in procedure
("mock tape-in") as a way to get a filled "blow-back"
database which you could then pick at in places where
you think parasitics matter.
Alternatively you could look in the PDK for cells that
look like pattern fill unit cells and go at it yourself, or
create one of your own (they tend to be simple, like
criss-crossed interconnect and contact layers at min
space, maybe with a border polygon for convenience).
When I did my own fills I distrusted the scripts and
did it by hand, using arrayed instances of the fill cell
provided. I often thought about how nice it would be
if, say, a pattern fill was also a useful on chip decoupling
capacitor. And I often made these and used them under
bussing to drive the density up, requiring less "dummy"
fills to meet rules.