Well, I got a chance, yesterday and today, to try a few cyanotypes using what I understand to be the rex methodology. Here's what I did, and what I got for results.
I used my existing cyanotype A solution, 200 g/L green ferric ammonium citrate in distilled water; the bottle was mixed around 18 months ago, and has been opened and small amounts used a number of times (in case that makes a difference). For each print, I coated 1 ml of this solution onto a 4 1/2 x 6 inch piece of Montval "Brut" (rough) watercolor paper. Prints were coated with a foam brush and dried under red safelight, some with a hair drier and others allowed to dry naturally.
Each print was exposed in mid-afternoon sun, yesterday in full "Sunny 16" conditions, today in "Hazy" to "Cloudy Dull" with an average of "Cloudy Bright" for most exposures. Full sun gave a dim printed out image (darkening of the citrate from yellow, as dried, to light brown) in about three minutes; dimmer conditions took five (this is at least two, probably three stops faster than the mixed solution consisting of equal parts of the above A solution and the B solution, 50 g/L potassium ferricyanide). The exposed print was then "developed" by brushing on approximately 1 ml of the ferricyanide solution (mixed at the same time, stored under the same conditions, and used a similar number of times to the A solution above), and either washed immediately or allowed to air dry before washing. Darkening of the exposed areas was immediate on contact of the ferricyanide solution, like a Disney animation of painting an image with a flat brush. I also attempted developing one print in a tray with about a pint of water and a few ml of ferricyanide solution added (roughly equivalent to 150-250 mg/L), and while some development took place, the image washed off faster than it formed (the solution was also dissolving both the exposed and unexposed citrate).
I found best results came from drying the print after "developing", then giving first wash in a tray containing about a pint of distilled water acidified with about 1/4 tsp of Kodak Indicator Stop Bath concentrate (to ensure against alkalinity in the paper, mostly). Second wash was in tap water until all yellow sensitizer was washed out. Prints are still darkening (oxidizing), but the best prints look as good as my best cyanotypes did at this stage (aside from brushing artifacts that appeared in development).
What I need at this point is a means of applying the developing solution evenly, without brushing away the surface reaction products or having them wash off in a tray of solution, both of which reduce density and muddle the image. I may need to optimize concentration of the solutions, but just this level of simplicity indicates the process can be two to three stops faster than the way it's usually done, with quality of results that's at least as good.
That said, what I have (so far) is nowhere near matching the claims made for cyanotype rex relative to speed, and far from being partly or largely independent of UV (hence why exposure only increased one stop when the light was two stops dimmer -- clouds reduce UV much less than visible light). Of course, the progenitors of cyanotype rex have invested hundreds of hours of lab time in developing the process (in parallel with chrysotype rex, which surely represents most of their incurred cost at $30 per 8x10), and I've put in (so far) about three hours, some of that spent watching sensitized paper dry.
We'll see what I get in future sessions...