PE, I routinely get cyanotype exposures in (North Carolina) summer sun that run from ten to fifteen minutes (in a contact frame, under a 9x12 cm negative developed to "normal" large format contrast). This is with traditional cyanotype made from the Formulary kit. That would suggest that C. rex would be too fast to expose this way with sunlight; I'd *have* to expose it under some kind of artificial source, lacking a neutral density filter for blue/UV light. Given the reported difference in speed, it might be that reports of non-UV exposure are simply because the stuff is fast enough to expose in the very low level of residual UV in light sources normally considered UV free (like common tungsten/argon lamps, near-sunset sunlight, etc.). Even so, that would make it practical for much of what I want to do with it, such as enlarge to C. rex (with a cold light, there ought to be significant UV from the tube in comparison to a tungsten enlarger source).
And as one who makes a fair amount of pinhole photographs, a multi-minute exposure doesn't seem terribly impractical (though it's an awfully long time for a portrait). According to those who've developed C. rex, the light level inside a camera (at an unspecified f-stop) isn't much different from that outside, which seems a bizarre result to me (but I've never yet put a light meter to the ground glass in my 4x5, it just *seems* a lot dimmer even with an f/4.5 lens wide open).
In any case, HTML, as far as I've been able to determine from my reading, that's exactly right -- coat with the green citrate solution (a little weaker than that normally used to mix cyano sensitizer, I think, though I wouldn't expect stronger solution to be a bad thing), expose until a faint image prints out, then develop with your choice of several solutions to produce cyanotype rex, chrysotype rex, kallitype, a platinum/palladium variant, or possibly the ferrogallate print you started this thread asking about (this is largely guesswork on my part, since the details of the rex processes aren't yet published -- the creators understandably wanting to make back at least some of their development materials costs on the $30/print gold process by selling the PDF and workshop slots before letting the process out for unlimited use from which they'll never see another dime). The originators suggest you get a "finer" print by using ferric oxalate instead of ferric ammonium citrate, but the citrate is faster.
I agree, PE, on your "almost get there from here" comment. There's a good reason silver processes "won" in the mid-19th century, and it wasn't because silver was cheap then (it was, and is, a precious metal, just not anything like *as* precious as gold or platinum) -- just that of the available chemistries, silver halide lends itself best to fine tuning from a bunch of different directions. I'd bet you could (with a century of research) do much the same with selenium, but it's both a lot more toxic than silver, and harder to work with (and a lot less common, at least in easily extracted form, moreso in the 19th century even than now); same for cadmium on all counts. However, for those of us on a limited budget, cyanotype rex is a lot more accessible than anything that requires buying silver nitrate in quantity greater than a few grams at a time (and the chemicals don't require hazmat releases, "statements of intent" for the "hazardous oxidizer", etc.).
OTOH, I have the cyanotype chemicals on hand, plenty of paper and negatives to print, and a darkroom to work in where I can avoid fogging the rex if it is that much more sensitive than "traditional" cyanotype (red safelight isn't going to fog the stuff, I'm betting, even if it's fast enough for enlargements). And for the next two weeks I have reduced schedule conflicts, so I should be able to experiment a good bit...