Is it practical to think of using spray starch as a sizing medium to prepare papers for salt printing ?
I don't really know but since all the materials involved in salt printing are relatively inexpensive, why not give it a try? Actually, a size is not really required when salt printing although it will help give the print a more detailed appearence since the image might actually be held in/upon the size rather than the paper fibers.
I've used spray starch as a size in the gum bichromate process after several layers have been printed. It helps refresh the sizing at that point, but I have not had good luck using it as the single initial sizing of the paper. For that I use either gelatin or canvas hide glue sizing which I later harden before printing on the paper.
I think the type of paper will be more of a concern with salt printing. The silver nitrate tends to react with organics in the paper and cause fog. If you find a good paper or discover that spray starch helps prevent fogging, I'd like to hear of those results.