One way is to register an aut-num if the routes are in a separate AS
and correctly indicate what other AS you peer with so others know
there are backup paths which are going to have to still work after
aggregation is done and not become primary paths.
If you don't have an AS then register the route in all the AS you are
multihomed to, not just one of them. This is gross, but I don't see a
good way around it.
In short, the only reason not to aggregate is to preserve correct
routing for a dual homed situation (or properly indicate a hole). The
means to indicate this is by properly registering the information in
the IRR. Then hope the proxy aggregator bothers to look there, but if
they don't then you have grounds to yell at them. Otherwise, they can