I'd use a circular saw, not a recip saw, because the latter is harder to control. Just set the blade right at the depth of the wood planks and slice a clean line the length of the joint, at the width away from the seam as the transition piece.

The circular saw won't be able to get all the way to the wall edge, so you'll need a Dremel or equivalent to slice through the last little bit.

Then just take the transition piece, slice off the tongue, cut it to length, drop it in the hole you cut, and face nail it down.

Pretty straightforward if you have the right tools, a steady hand, and a precise approach to measuring (need to get the width of the cutout exactly the width of the transition piece.

Thank you, Weedyacres. I can borrow a circular saw, and just picked up a multitool on sale so perhaps I can manage (or, I may try to bribe the cabinet installers into making the cut for me)

Depending if you have to rip a board down or not. Remove the tongue and the bottom groove too. Also dontforget to add glue to the bottom of the board before install b

You could also use a "T-strip" of brass or other metal.

They are made to cover the transitions between flooring by sliding into the gap - the arms of the T cover the edges.

Way easier.

Thank you,gregmills and lazygardens.

Lazygardens, I did think about those strips, but the ones that I have ever seen weren't all that attractive... maybe I will search a little bit more on the internet for brass T strip.

Schluter has brass strips.

I'm using them between the granite and the wood counters to seal the gaps