We did something similar due to very old/messy code, but not with a 3rd party service. We just built an Apache OutputFilter that recognizes text (also in alt, t

We did something similar due to very old/messy code, but not with a 3rd party service. We just built an Apache OutputFilter that recognizes text (also in alt, title, button value attributes) and replaces it based on a set of more than 200k regexps. Sounds slow, but works fine due to (tmpfs + mmap based) caching after some warmup. The first working prototype took about 4h. We'll never do it like this again though!