To be honest i wouldn`t bother with a xenon as like you say `how long before RROD`
The NAND needs to dumped first and see if the CB has changed or not, if the CB is still one that is exploitable then no need for R-JTAG as standard JTAG (switching diodes & a resistor) would be a better solution (quicker boot & cheaper). Though I don't know how long will a xenon last before it RROD's.