My immediate reaction is that this should be a feat that would let you spend channel energy uses to cast the appropriate spell. However, since the cleric already lacks meaningful class features, I could also see this as a class feature they obtain at, say, 4th level?
It doesn't seem overpowered or anything.
Clerics can choose to leave spell slots empty at the beginning of the day, then later in the day they can pray to prepare whatever spell they want in those slots. This takes a minimum of 15 minutes, and potentially longer if you're prepping a large number of slots at the same time (typically only the start of day preparation will be longer than the minimum 15 minutes). No need for house rules, just have your players keep a few low-level slots open at the start of the day for miscellaneous needs that may come up later.
Of course, you may homebrew as you wish, but I'd be leery of adding spontaneous spells to anyone's list. The cleric should just wait until the next day to prepare consecrate and deal with the unholy shrine at that time.
Eh, I'd pass on this. In addition to the reasons mentioned, it's awfully focused on good and evil clerics as opposed to neutral clerics. They already get more than enough focus, no reason to start tacking "make an area good and holy" onto "I just wanted to heal because Nethys is cool with either".
It's flavorful. In terms of game-breaking: not particularly. Likely won't see a lot of use unless the campaign focuses on the rise of a dark god or a holy war. I might even let them cast hallow/unhallow at higher levels spontaneously, and add bless/bane too. Clerics are powerful, but lack interesting class features. Giving them even a couple more spontaneous options for casting can't hurt in that regard. If power issues become a problem, perhaps require 2 uses of channel energy as well as the spell slot?
If your table is friendly to homebrew or soft rules, just use channel energy. Either deal damage to it, or give it a save, or do opposed caster levels.