That homemade laundry soap made with bar soap would be a nightmare in hard water. I don’t even want to think about soap scum in the drains and in my clothes.
I just use the smallest amount of detergent I can get out of the bottle, that works well. And don’t wash a garment after wearing it once if it’s not underwear. Invested in a lot of Merino stuff which manages to be comfortable even here in Florida and doesn’t stink ever. I can wear those shirts and just hang them back up.
You’d need to arrange the code ahead of time with whoever you were communicating with. I can imagine a few ways to do it, the number of letters in the words, the arrangement of the spaces between words, first letter of first word, second letter of second word, up through five words and start over, prearranged code phrases of course but nothing that would work with an unwitting recipient.