If I can make a suggestion, I would not put the content file on parity disks, especially if your parity disk and data disks are all the same size (rather than the parity disk being larger). My reason for this is that the content file on the data disks takes up a relatively small amount of space on each disk that actual data cannot occupy. This allows the available space of the parity disk to be just slightly larger than that of the data disks, ensuring that you can never "max out" the parity drive.
However, if you write that content file to all drives including parity, you could potentially end up maxing out the available space of the parity drive. Maybe it's inconsequential with the way SnapRAID is designed; I don't know how the program would react if there is no more space available for the parity file, but it sounds like an easy problem to avoid. Also, with three data disks, you'll probably have a sufficient number of copies of the content file that a fourth is not likely to become useful for rebuilding lost data.
But, for those who are stretching the limits of their storage, your point is legitimate and well taken.