Things can get messy if people sending you patches for something generated, rather than patches for the original source, though. Most such transformations aren't trivially reversible.
I understand the use case, and if people are frequently sending you patches for your HTML, you probably want to make HTML the authoritative source. Do people frequently send you patches for HTML that you author? SHPAML does not allow you to trivially reverse HTML patches, but it does allow you to manually apply them without too much pain, as it lets most HTML pass through unmolested. I actually source control my HTML output to be able to see diffs on the HTML, for what it's worth.