This was a previous life of mine. Just be aware that there's really a world of difference excavating archaeological sites and mass graves. I know this seems completely obvious, but a lot of people like the idea but end up not being able to hack it in reality. You see a lot of things that you can't unsee, on a lot fresher bodies than you encounter as an archaeologist.
You'll probably get accepted on the course - it's very unusual nowadays to be rejected from Master's study, apart from anything else, but you may not end up going further. Police in the UK tend to have a university lecturer on call for this kind of thing, rather than a permanent member of staff. To work internationally in this field, you'll usually need a couple of years excavation experience overseas, plus experience of some kind in post-conflict environments. Obviously, it's dependent on the wider situation - mass atrocity crimes don't happen on a schedule or predictably, obviously, and it's not always possible for international teams to go in - in Iraq recently, for example, because of the situation, it was primarily Iraqi teams excavating Yezidi graves etc. This is also very much of a push at the moment - getting the local population the training to enable them to do it.
I'm definitely not saying that you shouldn't start working towards it, but I think it's worth you knowing that it's not generally something you would make a living at, unless you are a super-Prof (and even then, they tend not to do this alone - most also have teaching posts.
If you decide to start the process, I'd recommend building on however much field experience you have got under your belt as an undergrad over this summer and during vacation in your post-grad, and factor in the likelihood of needing a couple of years contract experience thereafter. Once you've worked in the field and a) coped and b) shown you're a good person to work with etc, you'll be asked whenever something comes up. I've done some train-the-trainer stuff in the last couple of years, but I haven't done actual in depth, extensive war crimes stuff for around 15 years. I still get asked, though, whenever they need internationals, and despite now being completely away from archaeology. So once you're in, you will be in as long as you want - but as I say, it's really not for the majority of people even those who think it will be, so consider a Plan B also.
But feel free to pm me if you think I can be of help: it was bloody brilliant work, and I miss it very much. As I say, though, it's a lot more harrowing than you think could be the case, even if you have a brilliant imagination and loads of empathy, and I know far more people who've got into the field and discovered it's not for them than the people I know who have been able to do it - and there's no shame if that's the case.