You might find this package useful (it's mine ;): https://github.com/alphapapa/helm-org-rifle
Or you can install it from MELPA. I have plans to look into using external tools to find matches someday. git grep seems like the most promising, but maybe ag or agrep could work too. In the meantime, you may find the existing functionality useful. It wouldn't be hard to customize it slightly to do what you want, although you might like the feature to open selected subtrees in indirect buffers. Please feel free to give feedback on the tracker. :)
