You can definitely use more zero pages addresses, as long as they don't conflict with what the ROM needs.
That being said, if you are doing a microdisc based program, then you have full control of all the code in overlay ram, which means you can have you own IRQ handler which means that you can use 100% of the zero page for yourself.
Of course if you want to switch back between overlay and ROM, then it's going to crash horribly - except if you made a copy in ram somewhere else -.
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