Quote from Pham I have been thinking about it but didn't have time to implement yet. But I have customized Blogs module so I guess you have to customize the code (as I did with Blogs)

I have some basic knowledge that may help you in coding:
Table:

- ow_base_tag table : tag of everything in ow

- ow_base_entity_tag table : link between tag and entity (such as blogs, image, in your case should be forum/topic)

Those two are quite enough for data storing. As for php code:

- TagsInputField class : component representing html input

- BOL_TagDao 

- BOL_TagService

Hope that helps. 

Let me know if you are interested in exchanging the code customization. I am adding many cool/friendly feature into Blogs right now