It turnes out that it is absolutly trivial to force users of TortoiseSVN to add a comment while checking in changes to subversion, without messing around with pre-commit hooks on the server. Just add a property called “tsvn:logminsize” to the directories you want the restriction on, and set the value to 1 (or a larger value if you want to enforce a minimum comment length). You can have different values on different directories (just remember to set the property recursively, and then commit your new property). This is handled beautifully in TortoiseSVN, the “OK” button is simply disabled until an adequatly long comment is entered.
I found this via Anuj Gakhar.