In this example the first two notes of the bass should synchronise with the first two notes of the treble. And they do. My question was what to do in a more complicated situation.
But as Christian has solved the problem I think we can stop here, unless you're thinking of adding more functionality to the \cadenza construction, in which case I'm happy to oblige. I have other problems to see to (a new post will arrive shortly when I've written it)!
>> What I mean is that in some music the end of the 'free rhythm' section
>> may need to be synchronised with the other staves. For instance, at
>> the end of a florid passage for a soloist in free rhythm, the last few
>> notes may need to coincide with an accompaniment. The \cadenza
>> keywords don't seem to handle this. Would you like an example or have
>> I explained it well enough now?
> You posted an example. What in the bass of the example should
> synchronise where?