Well, I applied a liberal amount of epoxy resin to the cracks and on first sight it seems to have done the trick. The cracked portions no longer move relative to each other.
Not the prettiest of repairs as there are some glue runs - but if it survives my trip next week it'll suit me fine.
If not, then I won't be replacing the suitcase with another Rimowa, now that their prices are so high. They also seem to have stopped making 2-wheeled cabin trolleys, which is a shame.