I’m not quite sure what you mean when you say “to run as a droboapp.” Is it that you want the app to show up on your browser? Or to somehow have a virtual desktop on the drobo that you can connect to?
As far as I can see Calibre is an app with a graphical user interface. That, right there is a big no-no for droboapps. Theoretically speaking (and really, this is quite inside the realm of speculation) you could run a full fledged virtual graphical unix desktop on the drobo, and connect remotely to it whenever you need. The problem is that it would be so painfully slow that no one in a sane state of mind would use that.
Another possibility is if Calibre has some kind of separation between a so-called “frontend” and a “backend.” In other words, if the graphical app is just connecting to some server that usually runs on the same machine. You could then move the server to the drobo, but you would still need the client app.
Finally, the only realistic way I see to run Calibre as a droboapp is if it would offer some kind of web interface. Please correct me if I’m wrong, but I have not seen anything like that on their website.
Now, imagine that Calibre does offer a web interface. Then comes the complicated part, which is cross-compiling this thing. Cross-compiling difficulty is usually directly proportional to the number of dependencies. As it is, Calibre has a pretty damn long list of dependencies, and after a quick glance at them I’m not sure all of them are cross-compile friendly.