Yes, it doesn't require much maintenance, but sometimes you can be surprised. I started maintaining GNU grep 3 years ago because it was in a really sorry state. Some parts were almost rewritten to make it faster and more correct.
If I really want to create temp files I can use ed or use sed with shell redirection. Were it for some reason a requirement, I can avoid temp files, at least for the substitution (s) command, as follows:
sed -a '"$1"';H;$!d;g;w'$2
where $1 is some sed s commands and $2 is a file
If GNU grep was in need of repair and you fixed it, then I thank you. But I'm not clear on how that justifies any "extensions" to GNU grep.
Maintenance, at least to me, means fixing things, not extending them and adding more complexity and things that can potentially break or create incompatibilities across different UNIX's.
As I recall, AIX's default sed also does not support -i, but I no longer have access to AIX systems to test against.