According to the OCS deployment docs, you need a reverse proxy of some kind in order to publish out meeting related content, like your slides, documents, whiteboard contents, etc. It doesn’t matter if you use ISA (preferred) or anything else(Apache, etc) to publish this out, but how do you know your reverse proxy’s working ?
a) You could stop and start a load of meetings with whiteboard content
b) you could browse to https://reverseproxyurl.yourdomain.com/etc/place/null/slidefiles/blank.png and if your browser shows a nice empty page with certificate locks in place – it’s working. You may want to close down your browser between retry attempts. I also like to test with IE and Firefox to make sure everything works for most people.