I am a bit late replying, however I bottle pasteurise. Of course kegs are a bit bigger. A measure of pasteurisation is a Pasteurisation Unit (PU). The conventional target for Cider is 50PUs although there is a strong school of thought that anything over 30 PUs is enough for cider.
Now for the arithmetic... holding the cider at 60C for 1 minute will generate 1PU. but all is not lost as the number of PUs generated increases exponentially with temperature. Briefly, 60C for 1 minute = 1PU, 61C for 1 minute =1.4PUs, 62C for 1 minute =1.9PUs, 63C for 1 minute = 2.7PUs, 65c for 1 minute = 3.8PUs, 65C for 1 minute = 5.2PUs etc. So roughly, 65C for 10 minutes will give the same level of pasteurisation as 60C for 50 minutes.
Obviously heating a 20L keg will take longer than heating a bottle, but I guess it could be done with a big enough waterbath. I find with heating bottles in a 65C waterbath takes about 10 minutes to get them up to the bath temperature, then taking them out takes a similar time for them to cool down to below 60C. As anything over 60C results in PUs, this process generates around 50PUs during the heat up and cool down time. There is a post by Bembel (May 11, 2016) that goes into this, or you can search Google for HW del Veccio for the background research. Also have a look at Pappers' post at the top of the forum.
Hope this helps.