Cauliflower or celeriac makes the best mash imo. For either I cook with a small amount of chicken stock, add double cream or creme fraiche towards the end of cooking, then bung the whole lot in a blender (tip out a bit of the cooking stock before adding the cream if you realise you have too much). Plenty of white pepper (there should be enough salt from the stock).
You can - probably takes about the same time as potato wedges. I use lots of olive oil, sea salt and black pepper, and sometimes add some chilli flakes. (I add chilli flakes to a lot of things. There's nothing that can't be improved by the addition of chilli flakes, IMVHO )
I think its around 4gr per 100g so it scrapes into the under 4gr range. Cauli is a bit less but I find cauli mash can be a bit wet even well drained. Swede mash with cream and black pepper is lovely and it makes a delicious sub for roasties.