Perhaps I'm not following your point exactly.. but wouldn't the trading value of these hypothetical bitgold coins be linked to whatever value gold is trading at?
ie if 1 bitgold coin = 1 ounce .. then 1 bitgold coin (1 BGC) will always be valued at at least* whatever 1 ounce is trading at.
The value bitgold coins are traded at may be at a premium for the convenience factor of easy transferability provided by the blockchain based system..
and presumably this premium could fluctuate just as wildly as bitcoins. You've pegged* the base value only.
Now.. whether or not the underlying gold actually exists - if people trust it's backed by that gold then sure.. the lower value remains pegged to the gold price.
This is where I don't understand your claim to equivalence to bitcoins. Bitcoins don't have any such pegging system and so have no lower bound.
*regarding lower value peg: I say 1 BGC would be 'at least' valued at whatever gold unit it represents - but I guess there could be certain unlikely situations where it's less because people really would prefer to have the physical stuff..
e.g plausible rumours that the internet is about to collapse, imminent alien invasion,government crypto crackdown, a very effective virus running around stealing BGC or wikileaks releases the fact that your gold has been zapped.