Given the time/motivation, I'm sure the developers could adapt it to work on 1.3. But remember that WHDLoad's whole reason for being is to get games that don't work on expanded, 2.0+ systems to do so. I strongly suspect the original design logic years ago was that WHDLoad wouldn't be necessary for a 1.3 system.
I could be wrong. Browse through the older WHDLoad releases - maybe some of the really, really old ones do work under 1.3. But you'll need older versions of each individual slave to be compatible with an older WHDLoad, which means a smaller and buggier selection of installs. Many of the current installs didn't exist before v14.
