What's the convention for the symbols of enthalpies of formation, reaction, etc.?
I think you have already answered your question… IUPAC's usage should be a pretty good indicator of the proper notation.

There is, however, an even better reference than the Gold Book: IUPAC's Green Book, or Quantities, Units and Symbols in Physical Chemistry (full text here), on section 2.11.1 Other symbols and conventions in chemical thermodynamics (page 59 and following):


  (iii) Examples of the use of the symbol ∆
  The symbol ∆ denotes a change in an extensive thermodynamic quantity for a process. The addition of a subscript to the ∆ denotes a change in the property.


It says later:


  [∆vapH] can also be written ∆Hvap, but this usage is not recommended


The reason is that ∆Hvap could be understand as a change in Hvap, which it is not. It a change of H during a process, so the subscript should be on the ∆.