Hi
This is just an idea and I do not know if it is possible - it will need some research.
set the custom cells in hexadecimal, then
in Excel using a function or VBA code, convert the hex to ASCII text at runtime
for example:
62 61 63 6b 5f 70 72 69 63 65 in hex
is "back_price" in ASCII
Also, I think you can hide cell values / rows / columns in Excel. So perhaps you can do all your secret stuff in a hidden V/R/C.