no. In the short time I work at a z/OS-shop, they had to IPL twice. And the IPL takes ages...

Now, if you can live with the weird environment and your people know how to programm what is essentially a distributed system described in terms noone else uses: I guess it's still ok, given the competition is all executing IBMs playbook too.

Entire mainframe IPL, or just LPAR?

My understanding is that usually you subdivide into few LPARs and then reboot the production ones on schedule to prevent drift and ensure that yes, unplanned IPLs will work