There were a couple of standard "invisible inks." [0] I assume Invisiclues used one of them--most likely baking soda given they didn't use heat to reveal. No idea how stable either of those were as inks.
I think that Invisiclues is good idea, and that UHS format can be used as a computer file with a similar use. There are FOSS implementations of UHS such as OpenUHS and FreeUHS. Maybe someone will be able to rewrite the Invisiclues in UHS format. (I also wrote a UHS writer program in uxn. And, I had written UHS parser in PostScript; if you have printer with invisible ink (or scratch-off layer) then maybe it will be possible to use this PostScript code to make a program that will print out with invisible ink, too.)
[0] https://www.chem.ox.ac.uk/files/secretstoppers1pdf