This could be a useful feature if it was integrated into the UI. However, there is a workaround you can use to get this behavior in the meantime.
Let's say you want to highlight the word "strawberry" when it appears on any channel on EFnet, but not any other network. You could do that by first creating a custom alias in the remotes section like this:
alias hl-strawberry { if ($network == EFnet) { return strawberry } }
Then, you would set the highlight text to:
This would give the desired result.
Another common example might be if you want to match when anyone uses whatever nickname you are currently using, which can be done more simply by just setting the highlight text to