Sequoia Nursery grafted and rooted mini standards under mist for decades. The trick is using mist so the bud is kept moist until it knits with the stock so it doesn't dry out. The scion requires moisture either from being knitted to the stock or from being bathed by external moisture until the two pieces grow together. An unrooted stock is not flowing sap as there are no roots to absorb the water to be pushed up to the scion. Unless the process is conducted under pretty much 100% humidity so there is no possibility of the two pieces drying out before they knit together, I don't know how else it could be done successfully. Sequoia used a 3# coffee can with holes punched around the base about an inch from the bottom. Bundles of unrooted long whips with minis grafted to their tops were inserted into the can and placed under mist on a outdoor mist table which received strong, filtered light through the surrounding trees. During their hot weather, the bottoms of the whips remained in about an inch of water while the rest of the cuttings were maintained bathed in a mist of water which went off periodically. The strong light helped all the green parts to continue producing chlorophyll, feeding them until they grew roots and knitted together, at which point they were individually potted and replaced under less mist to gradually harden off and moved out into the sales area.