First of all, many thanks to gauthier for his excellent Maglev addon for pak128 (at https://simutrans.fr/doku.php?id=en:mlm_dev)!
I just noticed a mistake in the long block signals there: a maglev waiting at the presignal will only check that the track is clear up to the next station, not past it (but for a long block signal, it should check the way is clear past the station as well).
This seems to be fixed when I remove the is_presignal lines for the long block signals (so they have only is_longblocksignal=1, not also is_presignal=1).
The fixed .dats and .paks for the long block signals are attached -- I couldn't attach the merged .pak of all the maglev roadsigns as that was over the 64KB limit.
I hope this is useful for someone else!
Just updated the forum settings to allow attachments up to 2MB in size :)