Can the Lance's power guard block normally unblockable attacks?

In base World, no, the power guard is not a substitute for Guard Up. Trying to power guard an unblockable attack without Guard Up will result in getting hit as normal.

Iceborne added the ability to empower Power Guard, in which you maintain Power Guard for one second to actually block normally unblockable (as per the Guard Up text) attacks. Your body will flash once more so you will know when it happens.

This means that telegraphed unblockables such as Teostra's Supernova and Vaal Hazak's effluvial beams can now be blocked without Guard Up if you time your guard. However, fast unblockables such as Behemoth's shockwaves and Lunastra's ground explosion don't give you that much time, so you still have to rely on Guard Up for such unblockable speedsters.