Matt,
I think such a filter operator would naturally be in the core given similar support in most languages, and tiddlywiki uses filters to operate its fundamental logic.
However this can be dependant on the level of "bloat" to the core. If it requires a lot of bytes to implement it should be left out and placed in a plugin, perhaps in the tiddlywiki plugin library but documented on
tiddlywiki.com. It is traditional to present a plugin first, unless you can persuade the "developers" over in GitHub or here to include it in the core.
I expect a lot of people would not use this feature, but I can see why it would be valuable to some. Text based games, Flash card study notes, randomised questions, random stories, daily message, random post etc...
Regards
Tony