I found Axon's peak to dB conversion function and modified it a bit to show new peak value after applying track based ReplayGain. You could add a column with this code in your UI to see how much headroom (if any) you have before things start to clip after processing.
$if($and(%replaygain_track_peak%,%replaygain_track_gain%),
$puts(rgain,%replaygain_track_gain%)
$puts(valInt,$replace(%replaygain_track_peak%,.,))
$puts(numericVal,$add($get(valInt),0))
$puts(numOfZeroes,$sub($len($get(valInt)),$len($get(numericVal))))
$puts(isGe5,$ifgreater($substr($get(numericVal),0,1),4,-1,))
$puts(normVal,$cut($pad($ifequal($get(isGe5),-1,0,)$get(numericVal),5,0),5))
$puts(vMinus1,$sub($get(normVal),10000))
$puts(vPlus1,$add($get(normVal),10000))
$puts(frac,$div($get(vMinus1)0000,$get(vPlus1)))
$puts(frac3,$div($mul($get(frac),$get(vMinus1),$get(vMinus1)),$mul($get(vPlus1),$get(vPlus1),3)))
$puts(frac5,$div($mul($get(frac3),$get(vMinus1),$get(vMinus1),3),$mul($get(vPlus1),$get(vPlus1),5)))
$puts(frac7,$div($mul($get(frac5),$get(vMinus1),$get(vMinus1),5),$mul($get(vPlus1),$get(vPlus1),7)))
$puts(logSum,$add($get(frac),$get(frac3),$get(frac5),$get(frac7)))
$puts(logApprox,$muldiv($get(logSum),1737,100))
$puts(resultInt,$div($sub($get(logApprox),$mul($add($get(numOfZeroes),$get(isGe5)),200000)),10))
$puts(isNegative,$strchr($get(resultInt),-))
$puts(resultInt,$pad_right($replace($get(resultInt),-,),3,0))
$puts(resultInt,$add($left($get(resultInt),$sub($len($get(resultInt)),1)),$ifgreater($right($get(resultInt),1),4,1,0)))
$puts(resultInt,$pad_right($get(resultInt),3,0))
$puts(rhs,$right($get(resultInt),2))
$puts(difflen,$sub($len($get(resultInt)),$len($get(rhs))))
$puts(pint,$ifequal($get(isNegative),1,-,)$ifequal($get(difflen),0,0,$substr($get(resultInt),0,$get(difflen))))
$puts(pfrac,$get(rhs))
$puts(pfp,$get(pint)$num($get(pfrac),2))
$puts(gint,$num($get(rgain),0))
$puts(gfrac,$substr($get(rgain),$add($strchr($get(rgain),.),1),$add($strchr($get(rgain),.),3)))
$puts(gfp,$get(gint)$num($get(gfrac),2))
$puts(tmp,$add($get(pfp),$get(gfp)))
$puts(oint,$div($get(tmp),100))
$ifgreater($get(tmp),0,,$puts(tmp,$sub(0,$get(tmp))))
$puts(ofrac,$num($mod($get(tmp),100),2))
$get(oint).$get(ofrac) dB
,)
If you want to utilize this information use the preamp slider in the ReplayGain config appropriately. Note that even if you have headroom now, you may need it for future tracks.
Ps. The code gives only approximate value. I noticed almost 0.1 dB error when track peak was just a bit under 0.5. I don't think I have skills to fix that.