I have just tested an IO Pi Plus with the interrupt pins connected and set to go high when active. It appears to work correctly with the interrupt trigger showing on my logic analyser so in theory you should be able to connect all of the interrupt pins together and use a single voltage divider.
Having said that the one issue I can see is if one of the interrupt pins is accidentally set as an open-drain pin instead of active high you could have a direct short to ground from the other interrupt pins, possibly damaging the chips on all of the IO Pi boards.
A better solution would be to use a logic chip to act as a buffer between the interrupt outputs and the GPIO pin. You could use a CD4078 which is an 8-input OR/NOR gate. With an OR gate if any of the inputs goes high the output will go high so you can combine the interrupts from 8 I/O chips into a single output while keeping them isolated from each other.
You will need to run the CD4078B at 5V to make it work with the interrupt pins so you will still need a voltage divider on the output but this should give you a safer way to use all of the interrupts on a single GPIO pin.