Programming an Array of ATTiny
As long as whatever you're connecting the MISO, MOSI, and SCK pins to doesn't mind being driven externally, you can still program the ATTiny in-circuit - just break out those pins with reset to a standard 6 pin header and program them that way.

If one or more of those pins can't be driven externally (eg, because they're connected to another device that drives them and has no output disable), you'll need to add a tristate buffer IC between the MCU and the other device, or some jumpers you can remove when you want to program them.