Yeah, that's a good idea, I did consider it, but haven't tried it yet. Do you hook and look at the regex string before it's compiled, or do you hook in at the parsed regex AST level? (eg: regexp/syntax in Go).
In the go code where I did this, it was a little different, with a static pattern. Something like "(\w+) apple" to find all apple adjectives or whatever, but the regexp wasted so much time matching words before not apples. A quick scan for "apple" to eliminate impossible matches made it faster. This depends more on knowing regex and corpus, so probably less relevant for awk.
It's been a while since I've looked at the source code, but it is almost certainly already doing basic prefix literal optimizations.
The more advanced literal optimizations come into play when every match must start with one of a few possible characters or strings. Then you get into things like Aho-Corasick or fancy SIMD algorithms (Teddy).
Oh, regarding rure go, the bugs note about using int is inaccurate. Go spec says max length of any object can be stored in int. You can't build a too big haystack.
I got some pretty solid wins just by guarding some regex executions with simple strings.indexof calls.