Replying to my own thread in case others are looking for an answer:
I have found a way to do this relatively simply, by using PREPRO to prepare the ENDF tape as described in the IAEA publication linked above, and then writing a small python script to format the ENDF tape output from PREPRO to the specifications of the G4TENDL format. I imagine it would be relatively easy to adapt this to the G4NDL or other library formats, I’ve just been using G4TENDL.
I then use the code helpfully provided by this post: How to open .z files in downloaded G4Data files and check the cross section data - #6 by gunter
to zip the file into a zlib *.z format which is used by the G4TENDL library.
I have compiled all of these codes into a github repo here: GitHub - SZangi/ENDF_To_G4: A small set of codes to convert a file in ENDF format to G4TENDL format
I would strongly advise reading through all the documentation for the PREPRO commands used and looking through the code before using it as I am a novice at all this and there is a good chance I have made an error.