You could just run a simple script in something like Keyboard Maestro or Alfred. You could even automate the script so that it runs every hour, or day, or whatever you want. Most of the work has already been done by someone and the script was added into the “Script” section of the forum. Here is the link to it.
Now… In the other hand. @jessegrosjean, since you said that “It doesn’t make sense to set a relative date (such as 24m) directly on a task”, which makes perfect sense. Could the next version of TaskPaper change relative dates automatically when used in tags, but keep the relative dates untouched when using the search function?