KS 1.3 compatibility is the only missing "feature" but perhaps it isn't too difficult when sources are available. (at least I am going to try..)
I finally felt lazy enough to attempt this..
It compiles, all KS2.0+ routines replaced with wrappers (mainly using AROS library code). PFS3 even detects PFS3 formatted hardfile under 1.3 but thats all before whole AOS crashes (probably due to some memory corruption, crash is due to bogus values in some unrelated OS structures.. difficult to debug)
After some more debugging, I noticed PFS3 compiled under AROS amiga-m68k compilation environment does the same even if running under KS3.1. So there must be some kind of incompatibility somewhere.
I am using AROS compilation environment because it allows tricks like hiding all 2.0+ function prototypes, allowing 1.3 support with minimal changes to PFS3 source files.