Custom extension, what hook to use for UserDashBoard
By contact dashboard, do you mean the contact summary? In that case, there is a hook for that:

https://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_summary

Can you try to see if it alters the page you want?

Otherwise and more generally, they are two "catch all" hooks for the ui, one for page
https://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_pageRun
(likely the one you need) and one for form
https://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_alterContent 

With these two, you should be able to modify pretty much any content generated by civi