> Does this make any sense? Does it seem like a good idea?

For me that makes perfect sense.

What are the gains and what are the drawbacks?

Gains:

- no need to define two indices for fast support of min and max
simultaneosly;

- support for scrollable cursors (?)

Drawbacks:

- slower index performance when many pages are empty, but this can be
fixed by recreating index.

Anything else?