If this is working correctly you should see something like this is in the logs:
INFO server comment - VODTimedTextProviderBase.indexTimedText using file:/Library/WowzaMediaServer/content/sample.srt for captions (VOD:/Library/WowzaMediaServer/content/VideoEdit.mp4)
INFO server comment - VODTimedTextDelegateBase.narrowProviders[vod/_definst_/VideoEdit.mp4]: provider: vodcaptionproviderttml
I did test and asked around about this. I thought there might be an option to get this file from a web server, but I don't think there is. The .ttml files have to be in the content folder or below. The only thing I can think of is using symlink as subfolders of content location pointing to these directories.
You might also use MediaCache to setup different file locations, but you will still have to have the .ttml alongside the video,which probably doesn't solve your problem.