While a tooltip might appear handy on first sight, it would be a very bad idea because it could lock the UI for many seconds, depending on how complex the instrument you are hovering really es. OpenMPT needs to load the whole sample or instrument to know its bit rate, if it has one at all.

Can openMPT be set up to read the file then display the attributes in a new side box in the Sample Editor page?

I'll mark this as a duplicate of 0001389 where there is some further discussion and where some initial work was done (file size and modification date are now shown in the status bar). Some sort of dialog that has to be explicitly called by the user is probably the only way to implement this in a way that doesn't degrade application performance for everyone who doesn't need the feature.