decals on tyres ?

Decals don't stick to tires very well. You may have luck by coating the tires in future, but this isn't a surefire method. They may just end up falling off a couple days later.

the plan is to print the decal on white paper using a black background. Then cut out as close to the text as possible.

Will probably take at least two decals - one over the other - for it to look white but I reckon its do-able. Used this technique to put whiote text on a black fairing and it worked fine. My only concerns are getting the decal to stick and the fiddly nature of the job.

Waterslide paper... euh I thought that only came in white or transparent, tire printings are white, tires are almost black, how are you going to print that, since you can't print white with your home inkjet printer ?

this is for a kitbashed bike with custom decals that I intend to print on waterslide paper.

Doesn't Tamiya provide dry transfers for all of their tire decals?

just to be safe, I'd coat the decalling spot with future, and after the decal application, put future over it and over the future, a flat coat