MuseScore 3.6.2 & 3.7—Is there a way to make a Range Selection in the Piano Roll Editor?

The status bar below shows as a list-selection when a selection exists of multiple elements without a range-lasso.

If you're in control of the plugin's programming, alternatively you could take a list-selection (which is what exists from the lasso or ctrl+clicking inside of the PRE).... and get the appropriate elements (notes) from the elements container of the curScore:

QQmlListProperty < Ms::PluginAPI::Element > elements

These are graphical UI elements, so they're noteheads only and not ChordRests. You could create a range-selection within the plugin before doing whatever operation upon a range-selection, but you'll need the start and end ticks along with staff information.

curScore.selection.startSegment + endSegment only work for a valid range selection, so you'd have to take the curScore.selection.elements (which will be the list-selection) and get the first and last elements from there (check that they're notes), grab their ticks and their staffs, then you could synthesize a range-selection from the list selection. Notes don't have ticks, but they have Chord() method which has a staff. Grab that. Then call the parent() of the chord which will result in an element that will have a tick() (I've gone through this jazz myself before). Again, the chord itself will have a staff element. Those ticks and staves can be used to form a range:

curScore.startCmd();
curScore.selection.selectRange(beginTick, endTick, startStaff, endStaff);
curScore.endCmd();

After that, you should be good to do whatever the range-based operation is.

in 3.7 (Qt 5.15) If I remember correctly, I had to use startCmd() and endCmd() to get the range selection to work or something,. but in 3.6 with Qt 5.9 it didn't need it, whatever that means.