How much success have you had rooting old rootless Cattleya back-bulbs.

[Orchid Whisperer]

I wouldn't dismiss any root growth stimulant, as long as you know what's in it (I don't use Super thrive - nope, not me). Kelp extract, go for it. I've used KLN successfully (must have been a fresh batch, if not fresh, it won't work).

If the plant tissue is my capable of making roots, stimulants can work. Not all plant tissues can make roots, quite old back rhizomes are probably less capable. IMO, best hope on a back bulb is for the plant tissue to be hydrated enough for a dormant lateral meristem to start growing.