Not sure if this will help, but I just ran a plasticharge white on midnight blue Next Level N6210. Results were a lot better then I expected. After washing it was still a fairly bright print. It had a little fading/distressed look, but that was what my client was look for so it worked out. those were 50/50s though. I think rayon is considered a semi-synthetic fiber, so it may partially discharge and give you decent results.
IMO triblends are such a nice feeling shirt, it would be a shame to print a heavy plastisol on them. I don't print them often, but I'm trying broaden my horizons in terms of different inks and printing styles. I'd give discharge or plasticharge a try on them.