But the general complaint with these types of cases is that the locks are flimsy, offering no more protection that the fact that they LOOK like a lock.
So I started thinking that maybe a motorcycle trunk would have a more respectable lock. But of course there's the mounting issues, and the fact that most a probably too heavy for a bicycle anyway.
Has anyone ever tried this? Any suggestions on a good model to try?
Security of bikes is always fraught and is only ever solved if you can park your bike where you can see it. I don't think a hardcase would be any more useful or secure than a normal pannier.
Whether they are contained in a hard case or pannier doesn't really matter, and neither does the security (or otherwise) of the hadcase lock. It is easy enough for a thief to nick your bike entirely and then work on the "secure" hardcase at his/her leisure.
Doing the same thing with a moto is much much harder becasue they have far more sophisticated security (such as ignition and steering locks as well as alarms and immobilisers) which prevents taking the bike away in the first place.
It is always better to take all your stuff with you when you have to park your bike. In your positon I would prefer to go for a pannier which can double as a backpack or shoulder bag (most of them do now).
Tafi is right. With a bicycle, the only thing that locks do is keep honest people honest. There is no bicycle case or motorcycle case that cannot be opened or stolen itself by a competent theif.