Hi @michael,
You can definitely try dispensing your own higher-temp paste and using an oven. Like you mentioned, we have empty cartridges available on the store to fill up with your own materials. For any paste you use, I'd recommend a particle size no larger than T4 - otherwise you risk clogging the nozzle.
If you'd like to do the higher-temp reflow on the V-One, your best bet would be to use a tin-lead alloy, such as Pb40/Sn60 or Sn62Pb36Ag2, since these tend to melt at around 180-190C. Most reflow profiles will recommend a reflow temperature of 20-25C above the melting point, so the V-One heater should be able to reach the required temperature (max heater temp is 240C). Lead-free solders, such as the SAC family, have a higher melting point in the 220C range, which would likely be unsuitable for reflow on the V-One. Note that these would only work with a traditionally fabbed board - different solder alloys are not likely to work with conductive ink.
To deposit paste on the second side of the board, you'd likely need some standoffs and a different clamping mechanism to keep the board in-place during printing and probing, since you'll have components on the bottom. We don't have anything of the sort available on our store, but if you want to design a clamping mechanism, make sure that the V-One carriage can clear it comfortably when in the fully raised position.
It's definitely an interesting idea, and is something I'd really like to see it in action.
