WowAce

The moving part of the bar is the precise part of the bar telling us the current value. I'm not referring to the "maximum" part of the bar here.

I think it would be fun and new cool layouts would be possible if we were able to select the moving part of the bar as an attach point. The goal would be to allow us to add in an Indicator or Text that "follows" the current value of the bar as it depletes or grows.

New UI control: "Follow current value" (checkbox, only visible when a valid bar is chosen in "Attach to")
(It's very possible there is a much better word to describe this for users.)

Where the Text or Indicator is positioned in relation to the "current" part of the bar is still governed by the "Location" drop-down, but we'd want all of the standard anchoring choices in there: Top-Left, Top, Top-Right, Left, Middle, Right, Bottom-Left, Bottom, Bottom-RIght.
The "Position" drop-down is still useful for ordering multiple Texts / Indicators.

It's also probably a good idea to have 2 new 0-100 sliders so we can specify the "Minimum value position" and the "Maximum value postion". For example, in order to avoid multiple texts piling up on each other, we could specify that our Health text only moves between the 100% mark and the 50% mark, leaving the rest of the bar reserved for other texts.

CPU wise this would be a nightmare as you described it. It's certainly possible to use the edge of one of the subsidiary frames in the bar as an anchor. I've ended up doing it on accident in the process of fixing the icon for the cast bar leaving a blank spot in the background. However, in order to do the minimum value/maximum value position you'd have to recalculate the frame layout constantly as values change. Considering that a lot of our performance improvements of late have been eliminating layout calculations on fairly rare things like players leaving a raid group and that this resulted in dramatic improvements in battlegrounds. I don't even want to begin to imagine the mess what you're suggesting would entail. In order to do it we'd have to have a way just to reanchor just those texts as values changed or the performance drain would be massive. It'd hugely complicate the code.

Without the max/min value stuff it would be possible. But the text would scroll off the frame and be hidden or just show ... which isn't what I'd imagine you'd want.

I'll agree that it's a fairly vulgar and extravagant feature request. :)

Most of my LuaTexts are already written to hide themselves in the event that their value is maximum or minimum so I suppose the min/max slider thing is too much, especially considering the CPU cost (which I didn't think of at the time of posting this, sorry).

I'm now imagining the places I'll use this and it pretty much comes down to Player and Target frames only. Without the min/max stuff, a little extra LuaTexts magic might be fine. For example, we could have a HPLeft tag on the left of the end, and HPRight on the right, and just hide one or the other based on the value.