Again, with the suggestion above, this is not needed. This part of
the rule is that the bracketed note lengths must form a complete bar
in the current time signature. If not, an error is thrown.
Here, I go with David. Just think of notation used sometimes by
Richard Strauss:
5 4
4 4
which means that the first bar of a two-bar phrase holds 5/4, and the
second one 4/4 (cf. the `aria' of Octavian in the first act of
`Rosenkavalier'). To handle this, we need two beaming instructions:
\beaming 5/4 {...}
\beaming 4/4 {...}
to redefine the beaming structure for the two bar types. I think it
looks quite good in general to be able to define beaming structures
like this.