Guess what? Spoiler: xenForo has a few feature! You had to click a button to read this! You can make anything a "Spoiler" by highlighting the text you want to hide and clicking the Insert toolbar button, between the Media (filmstrips) and Drafts (floppy disk) buttons. Note that this is where you can add BB code for Quote, Code, and Strikethrough as well.
Unfortunately, Spoilers do not work in Tapatalk. You just see the hidden text right away. Kind of expected that. But, for those mobile users, the site is extremely mobile friendly, and the Spoiler function works on mobile browser! I know, it's not the same....