How about the people who want to actually get on a very busy train like the 345s in the COS during peak times, which would undoubtedly be displaced if the said amount of toilets required were to reduce capacity heavily?
Adding toilets to the 345s for a case of long stranding which isvery much a red herring, just doesn't make sense. There are other options you have toward Reading from the COS and toward Shenfield again from the COS that have onboard toilet facilities but also the speed for certain travels deemed long enough to "require" toilets on the Elizabeth Line.
Where would a toilet and its electronics and tank go? Assuming you put one toilet in Car 5, it would need to be a large, accessible toilet for wheelchair passengers to use but would undoubtedly make access to one of the three doors, let alone the walk-through space, dangerously cramped. The 720s have only two doors, as does most other stock with toilets, as they weren't expected to deal with the intensity of passenger flow the 345s do.
Assuming you do find a place for a tank and somehow logistically place the accessible toilet and one other toilet, it still doesn't cover that the trains' batteries will be much further depleted in a stranded situation of sufficient duration.
Adding more batteries on top of the added weight already from the toilets, their tanks, and electronics, plus the very reduced capacity for the options that already exist, in my opinion, doesn't make any sense, and I'm barely listing the issues.