Luckily, last week this placement played to my advantage: While searching for a wood filler better suited to this project, I found a post on the Rockler website that explained the different kinds of wood filler, and identified what I thought would be the right product for this job. But in the store, when I described the project to someone on the staff (identified by his name tag as — I kid you not! — "Woody"), he pointed me to something else even better: Mohawk Wood Epoxy Putty Sticks.
The Mohawk wood epoxy both filled the narrow cracks more effectively than the garden-variety filler I used three years ago, AND it's harder when cured. (Win-Win!) The main drawback in practice is that, for this application, you must quickly resign yourself to wasting at least 50% of what you mix: As with any epoxy product, you have a limited time to use what you mix ... but it's well-nigh impossible to mix only the amount you would use to fill a single fret slot. Heck, it's well-nigh impossible to mix only the amount you would use to fill two fret slots, and even that's a race against time — especially given the putty's tendency to stick to the X-acto knife, and the palette knife, and the latex gloves, and, now that you mention it, everything except the surfaces in the fret slot.
Here are photos of the slots filled (but not sanded):
If you look closely, you can see the difference between the five slots closest to the neck pickup (which I filled more precisely, using only an X-acto knife) and all the others (which I filled more quickly, using a palette knife). The latter group resulted in more stray putty on the fingerboard, but that's not a problem: the process of extracting the fretwire slightly raises the wood along the edges of each slot; when you sand those down to the level of the fretboard, you simultaneously remove the excess putty. But don't take my word for it; here are images of the fingerboard after sanding:
... and finally, here's the newly fretless instrument, restrung:
So, there you have it: Phase 1 (making the fingerboard fretless) is complete; now it's on to Phase 2 (finding a way to tie frets around the neck). I've procured an ample supply of artificial gut, in the form of 1.75 mm ABS nylon monofilament (normally fodder for 3D printers). The good news is that it definitely seems hard enough to work as fret material (in particular, it's harder than nylon cable ties), but it seems to require some persuasion (most likely, by a soldering iron) to attain a proper fit to the fingerboard. When I find a practical solution to this problem, I'll share the details here.