If it has bloomed or the flower buds have swollen then the sap’s flowing…  Still tight vegetative buds closer in may work.  We’ll often get scions from the ‘day of’ the event, and will graft them to the dormant rootstock if asked…  It’s not ideal, but you’ve nothing to lose trying.  

If the scion doesn't take, you’d still have the rootstock and could allow it to sprout and send up a ‘replacement trunk’ to be grafted to next year with dormant scion wood (or budded late this summer).  There’s no need to chill the scions, the sooner the better as far as using them.  

With regard to the HOS ...those folks have not only become longtime friends but are a forever source of inspiration - it takes everyone, including you ~

Interesting on the Shield Budding, Rooney.  So, perfect timing is needed for a swelling scion bud.  In this case, the rootstock’s still dormant, so by the time it’s sap begins to flow, the bud will have burst.  If the sap’s already begun to flow on the ‘budding stick’ (scion) it would be seriously interrupted if cut and stored until the rootstock sap flow ‘caught up.’  Had the rootstock been planted out (growing at the same pace), there’d be a match in sap flow ‘about now,’ thus the shield graft would be a viable option.  ...some of this is just me thinking out loud… but that’s what I’m coming up with.

Sometimes (at the grafting tables) we’ll ‘pop off’ the main vegetative bud if it’s pushed too far, make the graft, and allow the adjacent or secondary buds to sprout.  'We' had someone dispute that scenario at the event Sunday, saying the sap flow interruption (due to the graft) would starve or kill the scion … but, having popped off the erupted bud, there’d be little to no loss of ‘moisture/ sap’ due to it’s continued growth or ‘pull.’  

Problem is, we get little to no feedback on these grafts, having made them at the request of our ‘client,’ and something few grafters would have done for themselves