myfield:"(" does work, but
[…] searching on fields requires an ‘exact match’ by default.
You can use myfield:"*(*" instead.
I’m currently working on overhauling the search syntax. Since what you’re describing isn’t a bug, it won’t be changed, but if all goes well you’ll also have the option to escape parentheses with a \ at some point.
However, if you find any other issue with the search syntax, please let me know.