I have seen the fonts example on the FOP website -
http://xml.apache.org/fop/fonts.html

I have implemented a batch (.bat) file which opens the command prompt and
then creates the .xml file for the font - here is the command:

However, i'd like to be able to do this from a JAVA class / .jar file -
passing only the ttf font filepath to the JAVA class.

PRocessor specific questions are best asked on the processor
specific lists.
In your case, look at the "embedding" page on the FOP site,
specifically
 http://xml.apache.org/fop/embedding.html#config-external