(part 2) More research.

So it seems that "Inkjet Transparent Film" is usually also PET. There seems to be many options for getting it in A3/A4 sheets, but there aren't many places I've found that has a roll of the stuff.

• https://all-pervading.com/products/33cm-width-high-quality-inkjet-transparency-film-for-screen-printing-30m-roll?_pos=9&_sid=7fea93aa0&_ss=r

From this video, I've learned that "waterproof" versions of these kinds of film have more of an emulsion substance on one side than the "non waterproof" version. So far I can't find information on what this material is.

I'm still on the lookout for cheaper sources of PET in a roll. A 50 pack of A2 laminate pouches (+ £8 shipping) would only cost 96p/sq m. The 33cm x 30m roll of transparency film (+ 20EUR shipping) seems to be £5.05/sq m, implying that there is still room for a lower cost roll. 

The roll also has to be split somehow if the shorter side of the LCD is to be used (164mm for the M3 Max + maybe 20mm each side). Even for the M3 Max screen of 298mm long, a 12mm gap between resin exposure zones, and perhaps 1m lost on each roll (so 28m of the roll is usable), a 30m roll would be the equivalent of 88 FEP / nFEP sheets that can be swapped electronically with the press of a button, and that's with the 13cm wide roll cuttoff, which could be used on a Photon M3 or Mono 4K (175 or 194 equivalents respectively). It means that, while the film is a consumable, it would be a very slow consumable with little downtime.

That's only on the assumption that PET works though, as the comments of those that have tried it aren't good. The VLM based design might be more forgiving.