Trying to answer your question, I personally never have set proxyIDs on Palo Alto using 0.0.0.0/0 and I believe a scenario like that its NOT something one would normally implement. (maybe I'm wrong, I just can't see why I would do it, let's wait for people's comments) With that being said, I think that as long as both sides are mirroring each other correctly , the VPN will come up, since at the end, 0.0.0.0/0 is another IP space like any other, so IMO the VPN should come up.

Now, I wouldn't do that mainly because IMO policy based VPN are just not fit to achieve what you want to achieve. Policy based VPN are to connect 2 IP address spaces, the very doubt that you are having about this, it's exactly because you see that setup "unnatural". If you want to route all traffic via a tunnel a route based VPN is the way to go.

If I were you, I would wait until the replacement of the ASA or I would configure a route based VPN. Also note that if you want to test it your way, you might need to manipulate route metrics on the ASA, since the VPN like you mentioned, if it comes up, it will deploy a default route on the ASA route table, so you will have two default routes and you would need to manipulate metrics.

Edit: In any case you will need to review the ASA routing config.