Surprisingly, you don't have to be 5.0 or 5.5 level to do and pass coaching certification. Of course, you need to be able to teach people all facets of the game so you need to be skilled enough in them before going for your coaching test (perhaps 4.0 would be the lowest). Then, it just depends on who you do your testing through and what level you go for. Different organisations have different requirements and different countries have different levels of qualification. If you're just a good to average club player you might be able to qualify as a developmental coach. So you would be okay to teach at grassroots level or to rank beginners. I mean, be honest, if you're not obviously better than me why would I listen to you? So you could do it if you stayed realistic.
Look at the USTA site and you'll be able to see there are a couple of different levels of qualification. If you can fulfil the requirements then you can go for their tests.