That's actually already possible in FE7, I've actually done it in FE6A; let me find the events.
ORG $D3226C // This is what you'd put instead of the death quote's text pointer
IFCA 0x00 0x05 Roy // If Roy kills Erik
ENUT 0x0E // Trigger 0x0E
ELSE 0x06 // If anyone other than Roy kills Erik
Then in my ending event I had this:
IFET 0x01 0x0E // If Roy killed Erik
ITGC Roy Sybaris
The way I have it set up, if you have Roy kill Erik, you get an exclusive item along with a different death quote and conversation afterwords. If anyone else kills Erik, you don't get anything special.