Jonathan,
I don't think anyone knows the answer yet. I would assume for LIV docs that you input it in MIRO/MIR4 so that the detailed invoice XML extract program (which has yet to be released) would extract it from there but putting it instead in FB03 may work. We have to see how the program will work. If LIV docs can have the value stored in the Accounting text and also have the invoice XML program extract it that would be nice as the work described in 2075584 is a "consulting note" and SAP states it isn't supported so having to do fewer changes and having them supported would be ideal.
When SAP releases the invoice XML extract program I guess we'll see!