I've just taken a look at the GUI files and I can't see a way of doing it. It looks like it's hardcoded to use figures, but someone more familiar with interfaces might know more than me.
The display is not hardcoded as I tried modifying this some times ago but the names of each part (in which textures are set : cf .CFG files) seem to be hardcoded. What you can do is to change settings modifying coordinates and eventually modify setting dimensions. If you do so, be careful to resize your textures to fit correctly. The coordinates columns in the .SPR file must reflect these changes to be in the modified frames.
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ST:A2 Modding, Mapping and Editing 
Special Energy Display 
Assimilate this! 
Linear Mode
