NOTATIO

No, it wasn't you who were dense, it was me. It had slipped my mind that you can double-click on the boxes in Beam Extension Tool and select which beam(s) to extend. I'm sorry!

But I think I might have found an even better solution to the problem all the same. Using 'Extend secondary beams over rests' undeniably saves some time, and is safer, because you don't risk the extended beams to be too long or too short if the layout changes.

To be able to do so and get rid of the weird 16th beam stub in B1, you may
- hide the rest in Speedy, which makes the stub revert to normal (B2), and
- replace it with a 16th rest as an expression (B3).

That's an interesting solution, but one would have to replace all the affected rests throughout the piece. So one would have to decide which was more practical. I checked into other possibilities in Finale, and the one I show above is the only one I have found that works. The steps are:

1. input 2. break the 1/8th beam 3. level the beams 4. extend the beams with the Beam Extension Tool

Also workable, I think.
I would still use 'Extend secondary beams over rests', though, for three reasons:
A. The process may be made much simpler, like this: 1. Input with 'Extend secondary beams over rests' selected.
2. Break secondary beams at x.
Done.
B. Working without manually extended beams is safer, as I mentioned before.
C. Replacing rests in my 'B' rhythm takes very little time, once you have created an expression for the rest.

EDIT: Obviously the context has to be taken into account. If there are very many 'B' rhythms, and very few instances of 'Hector's rhythm', your method may be preferrable.

Thank you all, once again, for the expert advice! It has been a great help!
Cheers,
Hector.

You are very wecome. Hector. One other possibility:

That's an elegant idea!
Many thanks
HP.