From what I can read in the config scripts, it looks as if only percentage bars are supported, but I would like to get that confirmed (I may have overlooked it).
To explain what I mean I'll give descriptions of the three basic types of bars:
1) Percentage bar
The percentage bar usually has 100 pixels and is defined by the maximum values. If the maximum value is for example 200, then each pixel stands for two points - if it's 5000, then each pixel stands for 50 points.
This is the most commonly used type of bar, but it needs its maximum value defined and the larger the values, the bigger the steps. If Maximum HP is 5000, then the bar would not differ between 30 HP and 45 HP.
2) Linear bar
Each pixel stands for a fixed number of points, let's say one pixel for 10 points. If the maximum value is raised, then the bar becomes longer.
You can see that form of a bar in some games as a visual representation of gained HP, and it has the advantage that the visibility of the points is good - but if the values go too high the bar won't fit to any computer screen any longer.
3) Logarithmic bar
This type of bar is rarely used because it doesn't make any sense for HP or similiar values - but it is very good for representing timers.
The bar is devided and usually color coded. The first part (for example 30 pixels) is green and one pixel stands for one point. The second part is yellow, and each pixel stands for 10 points. The third part may be red, and each pixel would stand for 100 points.
Such a bar can represent for very high values (in fact, the bar should never be full/max no matter how high the value goes, if needed add a fourth segment with 1000 points per pixel) while retaining the ability for exact point identification when dropping to low values (which is why it's often used for timers that can go up to any value).
I'm especially interested how much work it would be to implement the third type - theoretically a logarithmic bar can be created by placing several linear bars with different factors, but that is more work. And if linear bars also aren't supported by LUNA, then it would probably be easier to program a dedicated HUD instead of using LUNA as a background.