forgot to qualify— that lack of sap flow kills grafts quickly only in warm weather when branches are supposed to leaf out, or already leafed out.
admittedly my explanations were scattered over different posts, but i did mention that heavy sap flow is more of a mechanical issue, and not a nutritional/physiologic issue. With an overflow of sap, the budwood is pretty much sitting on liquid(sap). The sap will sustain the budwood, and the budwood will leaf out, and callus will form, but this may delay cambial contact if budwood is not secured/splinted, especially if the budwood is already growing long stems. This is one of the reasons why bark-grafts tend to get stripped down like a banana peel from the rootstock, because the rich sap encourages rapid growth, but xylem production is relatively delayed … An omega-type same-caliper graft would be the best for this scenario, plus extra splinting to minimize lateral movement.
as have mentioned on one of my posts, budwood will have some stored carbs/proteins/minerals/water within the stem, and will callus and leaf out when placed in a warm location, but this of course is quite limited relative to the bounty stored within the roots, and will ultimately need to be grafted to rootstock if callusing cannot produce roots.
So yes, callusing will occur upon local application of warmth even with minimal or zero sap input from rootstock, just as an entire budwood will callus at the base and bud out from nodes once removed from the fridge, but it is imperative that a steady source of sap be initiated later.
hot callusing is employed to heal open wounds rather quickly before sap starts seeping out(a magnet for infection) and likewise to have de novo xylem/phloem develop as early as possible. But in aseptic conditions with secure splinting of grafts, plenty of sap is a good thing.