Is it possible to use openocd to program an atmega using an AVR ISP (mk2)?
I don't believe openOCD even supports the Atmel AVRISP mk2. The list of supported dongles is here. OpenOCD uses the JTAG communication protocol to communicate with, set breakpoints, and control its target processors and FPGA's (IEEE standard 1149.1 in case you care). The Atmel AVRISP mk2 does not communicate using this protocol (in general). In limited cases it supports programming using something like it, but it isn't fully 1149.1 compatible -- so it isn't even likely that someone will add support for it to OpenOCD.