JAD - My one concern with this would be that this glue is designed to expand while setting. That's how it works on a loose chair tenon.
It may not be an issue because:
- You're using a very small amount of glue in a fret slot
- Most fretboards are a made from very hard wood
- Expansion would be in the direction of the endgrain where you would have the most compressive strength
Now, it's also possible that these glues are hygroscopic and achieve some of the expansion by water absorption from the atmosphere and adjacent wood. You can understand how that might become an issue.
And ultimately, do you really need anything fancy or experimental here? If a fret is loose, I think a standard aliphatic gap-filling glue (ie. Titebond), or CA, would be sufficient.
best,
Rick