Can I repurpose the ISP pins in the lower right corner of the Melzi board?
In theory, you can; but, you may need those pins to attach an external programmer to bring your system back to life when playing around with the code.

Here are a couple of articles that could help if you still want to pursue that path:


Can SPI programmer interfere with other components on same pins?
Can I reuse ISP pins for other tasks?(Target: Atmega164p)


Another option (the one I chose) is to just buy a RAMPS board set.  I got one on ebay for $19 and it have lots of more options for IO.  That way you can play and still go back and plug in your stock Melzi and print whenever you need it.

I got all this for $40 - boards, display, cables, power supply, and even shipping


Here is a really nice detailed description of converting a Duplicator i3 from Melzi to RAMPS.  The process would likely be very similar for your printer.

The biggest challenge will likely be setting up the firmware

BTW, what printer did you get?