I'm giving it a go. I don't think it can hurt and I know some people who swear by it. There's a demonstration by a midwife on YouTube (with a manakin, not a pregnant lady!)
If you google it there is also a leaflet from John Radcliffe in Oxford that comes up as a top hit. That advises same approach but suggests just stretching for about 2 minutes before massaging to let it numb a bit. This has made it more comfortable for me.
The studies show it is most effective in first time mums over 30 but who knows? If I don't do it and tear I won't know if massage would have helped. If I do do it and tear I'll never know if the tear would have been worse had I not done it..... So, back to the it can't hurt philosophy!
I can't reach from the front with my thumbs, but i can reach round my back (one hand at a time) with my fingers - then pull back towards my bum. I still put one foot at a time on the edge of the bath. If you wipe your bum front to back by reaching round the back - its a bit like that.