The infamous 2DW zener have lower noise, and will be much easier and better use then chemical cell.
Also you might even connect JJA itself as reference in HP 3458A, but you so pin-focused on noise alone, don't forget about reference gain stages on ADC, which convert 7.2V into +12V, -12V and +5V. These are the actual reference levels which used by ADC, not the 7.2V.
But either way, I'd suggest to build some boards and just go ahead with experimenting. You will find many not-so-obvious interesting stuff, while playing with 3458A A1/A9/A3 stuff.
No amount of chat on forum will replace actual hands on experience.