Can I use southeast asian fish sauce as a marinade?
No, it's not unusual at all. See this page from Vietnamese-American cooks for examples where nuoc-mam is used to marinate chicken, or just type 


  nuoc mam marinade


in Google :)

There are a lot of examples, including fish, chicken, beef, or even mango marinated (as well as dipped, of course) with nuoc-mam (and most of the time lemongrass), and a fair share of them are recipes by 'natives' from South-East Asia, not only fusion food addicts (even if you can be both!).
Some salads like goi tom or goi ga also involve nuoc mam marinade.