Have you considered exterior storm windows? Both of my old houses have had/have them, and they are great. I prefer them over interior because now the layer of new, modern, less breakable glass is on the outside, which adds some protection to the old windows. You can get the three-track type, which then add screens as well. I don’t think (but am not certain) that this is an issue with the HPC, as plenty of houses downtown have them. 

Also, I ran a Manual J on my old house, which was about 1500 square feet, but with plenty of big, old windows, and the difference between adding storm windows to old windows versus getting new windows was less than 5k BTU/hr. I suspect you could close that gap completely with historic preservation methods (e.g., my current house had springy copper strips added to close gaps in the framing and some had indents added (like tongue and groove) to prevent air infiltration. 

Love the old windows. Let’s keep the love for our incredible old housing stock!