Any use for Palkia in PvP?

Not particularly. Being stuck w/ 75 energy moves doesn't really give it a solid reason to be used over the other Dragons. Its typing gives it interesting properties such as Steel resistance and not being weak to Ice, but it can't make good use of these because it can't outpace these attackers due to the aformentioned charge times. Stick with Giratina/Latios/Legacy Dragonite in Masters.

Now Niantic can throw a curveball and give it Surf, but with the current gamemaster moveset, Palkia's pretty outclassed. It's not a bad Pokemon in a vacuum, but it has very strong competitors. It's extremely threatening at full charge, but as for actually getting these moves off, I'm not entirely sure how realistic that is.

This is actually the first time I've read about someone saying Rayquaza being good in Master League? I've tried it several times but I've always found it to be pretty poor in performance compared to other dragons like Giratina, Dragonite, and Latios

unlikely to dethrone girataina, its the dragon master in master league. It has the superior moveset and stat distribution that is suited extremely well/perfectly for PvP

Rayqyaza isnt particularly good in masters, its stats are geared far to heavily into offense, which while great for raids, isnt so much for PvP. The other problem is its moveset, again, great for raids, but with the mechanics of PvP, its an inferior moveset compared to Dragonbreath/dragon claw, which Giratina has.

Palkia, could be better than ray because of the better fast move and bulk (though ray's outrage is better than palkia's DM in PvP), but not better than Giratina Even dialga, the dragon that isnt weak to dragon will struggle against giratina simply due to the spamability of dragon claw. Its unlikely anydragon that doesnt have the dreaded DB/DC combo can take the throne from giratina.

Its PvP usage likely will be a hard fire or water counter, however I haven't heard much about fire being prevalent, and the big maters water types, kyogre (who likely will loose to palkia) and gyarados, who doesnt go down without a fight and can potentially have dragon breath or dragon tail on legacy users, or most all now adays have outrage. If going against kyogre, this likely will be palkia's win, forcing the, to switch, while gyarados is much more about of they have either the legacy move or are carrying outrage. If the sets Dragon breath/Outrage, its likely over for palkia.

Dragon Breath itself can hurt opponents a lot if not resisted. Palkia can be put at the last place in PvP when opponents are usually running out of shields.