TL;DR: This has been possible forever. You're certainly allowed to do it (we can't stop you) but I'll present the following caveats as reasons we haven't and won't document this:
- You can completely break Perspective, pretty easily, with this
- We don't want you to have to know HTML or JS to develop in Perspective
- We don't want people doing silly stuff like trying to dynamically modify these
- Anything you do use this for should be considered unsupported. If we ever broke this functionality, it would likely be because we offered a supported, first-party solution, so I wouldn't necessarily worry about it dropping out from underneath you, but also, don't even think about contacting support if you do use this and something goes wrong.