Alex,
Here's my suggestion.
If you put that multi-homed customer in a larger aggregate (have them
pick one of the providers and allocate from their address space) all
of the providers must then announce the more specific. Some providers
will block the longer prefix. The longer prefix will be preferred and
traffic will avoid going through those providers that block it. This
might cause longer or suboptimal routing for the longer prefix.
Providers everywhere will have either the shorter prefix or both, so
full connectivity would exist.
If the multi-homing is sufficiently localized within the topology (for
example, multiple providers in the same region or country) there might
be a chance to draw an aggregation boundary around the whole thing and
block the longer prefix outside of that locality and avoid the
possibility of suboptimal routing due to long prefix filtering.
Curtis