There is no direct way to check the range,
I think the best way to do such things would be comparing Coordinates with
http://wowwiki.wikia.com/wiki/API_GetPlayerMapPositionsince you're talking about PvP, Coordinates are available both in the world and battlegrounds.
(I'm saying that because it's not available in dungeons, that's why CThunwarner is based on interactranges)
I could imagine creating a way to calculate the range for players you have in target, it should be kinda accurate.
But this method would be impossible to do for players which you don't have in target.
I'll take a look at it this evening/weekend (if I don't forget it)
How would you like to have the range displayed to you? a simple "Range: X Yards" text element which you can drag whereever you like it to be, and which is only visible while you have a target?
EDIT: the function I mentioned can not give Z coordinates, which mean the range I could calculate would not be accurate if eg your target is on a hill above you. It could only calculate the accurate range between you and your target if you're on more or less even ground