Might not be that hard, have found some of my old notes/links.
There is a pastebin for a working Guilty Gear XX Reloaded Pic source pulled from mame.
http://pastebin.com/Wn33TXMk
From my original Link the Pic Key for Guilty Gear XX Reloaded is
3EE92354DC68642F
And the Filename is
BGB
It could be you just need to swap the details for the key you want to use
In the Pic Source File the AKeyCode and BKeyCode would need changing on lines 990 to 1005 to replace the hex entries with the new ones. The key is 16 characters long the first 14 go in the AkeyCode section after the 0x33 part and the last 2 go in BkeyCode after the 0x34 part, if you compare the file with the key above it should hopefully make sense, so 3EE92354DC68642F becomes 0x3E, 0xE9, 0x23, etc.
Then you would need to update the filename part starting from line 1013 that currently says 5BGB.bin to match the filename listed on the link of details.
The AYJIOANSWER shouldnt need updating as it should already be set to GD-ROM and not netdimm.
After that it should be compiled and written to a pic chip and that should be it.