[Orgmode] target-links (backlink) relation and global radio targets

[Maxim Loginov]

I'm wondering if it is possible to see the all links from the target?
for example, I have project B depending on project A like this:

#+SEQ_TODO: TODO NEXT WAITING DONE
* TODO project A
# <<project A>>

* WAITING project B
  project is waiting completion of [[project A]]

when the project A is finished I'd like to check which one is depends on it and
to switch it to state NEXT in order to start working on it. it would be nice
to execute some function when cursor is on target <<project A>> and see
the sparse tree (headlines + some text) with all points where the link to 
the target occurs. is it possible to implement this with special agenda view?
please give me a hint

one more question about radio targets:
is it difficult to implement global radio targets list for all files referred 
by  org-agenda-files? and update this list for example every tame agenda view
is called? could anybody write a code snippet? 

thank you and best regards
Maxim