OK, followup to my last “PS”. Even though no one asked, but I’ve got some upvotes:

Here’s a short function I didn’t test extensively (can’t post the one I use, because that one’s specific to our work and is intertwined with more of our functions like checking a file’s extension and properties to automatically determine its field order and if it needs to be deinterlaced):

It worked in a few short tests, so it should be OK, but if anyone notices a problem, feel free to add to that:

function tff_for_all(clip c, string "fieldorder") {
  #Usage if source is TFF: tff_for_all("tff") | else if BFF:  tff_for_all() or tff_for_all("bff")
  fieldorder = default(fieldorder, "bff")
  c
  fieldorder == "bff" ? propSet("_FieldBased", 1).AssumeBFF().DoubleWeave().SelectOdd() : last
  propSet("_FieldBased", 2).AssumeTFF()
}