On hashtags, I've removed the Reply Only To Sender checkbox and replaced it with a dropdown featuring all possible reply to options (all hashtags with the existing Reply Only To Sender were converted to the new scheme). Also, I've made it so that you can override the group's reply to on a topic by topic basis.
If a topic has a hashtag that overrides the group reply to and the topic is also set to override the group reply to, the topic setting takes priority.
If there are multiple hashtags for a topic that override the reply to, it is not defined which one 'wins out'.
In the /topics page, the dropdown displays the 'effective' reply to, as either set by a hashtag or on the topic itself. This can lead to some weird behavior: if you have a hashtag with a reply to, setting 'Use Group Reply To Setting' with the dropdown will never 'stick', because the hashtag is forcing a reply to change in the absence of a topic reply to setting. (You can go to the Edit Topics page and remove the offending hashtag if you wish).
If you go to the Edit Topic page, the Reply To setting there is the setting for the topic itself, not modified by any applicable hashtags (ie this is different than the dropdown on the /topics page).
This is a bit complicated, I know. Please let me know if you see any bugs or have suggestions for improvements.