I don't know if you will consider adding this in your mod, but i often check if i'm rested or not, and your addon was too good to keep the vanilla xp bar just for that.
It's probably highly unoptimized code but it works (i don't know what the tooltip would do for a lvl 40 character though).

In the text of the bar, adds " R" if rested :
function snt_bar.xp_upd()
   if (GameData.Player.level == 40) then snt_bar["xp"] = "LEVELCAP" return end
   local xp= GameData.Player.Experience.curXpEarned
   local xp_need = GameData.Player.Experience.curXpNeeded
   local xp_string = string.format("%d : %02d%% [%0.1fk]",GameData.Player.level,xp/xp_need*100,(xp_need-xp)/1000)
   if (GameData.Player.Experience.restXp == 0) then snt_bar["xp"] = xp_string return end
   snt_bar["xp"] = string.format("%s R", xp_string)
end

In the tooltip, adds a line with remaining rested xp :
function snt_bar.xp_tooltip()
   local xp= GameData.Player.Experience.curXpEarned
   local xp_need = GameData.Player.Experience.curXpNeeded
   local xp_rested = GameData.Player.Experience.restXp
   Tooltips.CreateTextOnlyTooltip("snt_xp",nil)
   Tooltips.AnchorTooltip(Tooltips.ANCHOR_WINDOW_BOTTOM)
   Tooltips.SetTooltipColor(1,1,unpack(SNTBAR.colors.xp))
   Tooltips.SetTooltipText(1,1,L"EXP INFO")
   Tooltips.SetTooltipText(2,1,L"XP")
   Tooltips.SetTooltipText(3,1,L"NEEDED")
   Tooltips.SetTooltipText(4,1,L"REMAINING")
   Tooltips.SetTooltipText(5,1,L"RESTED")
   Tooltips.SetTooltipText(2,2,towstring(tostring(xp)))
   Tooltips.SetTooltipText(3,2,towstring(tostring(xp_need)))
   Tooltips.SetTooltipText(4,2,towstring(tostring(xp_need-xp)))
   Tooltips.SetTooltipText(5,2,towstring(tostring(xp_rested)))
   Tooltips.Finalize()
end