Can't you list it as some kind of offshore trust account, or foreign currency holdings, If you declare bitcoins I don't think they would want to acknowledge it, they may not see it as such an asset because of the volatility of it (if they do recognise it they may undervalue it).
Banks don't normally accept your word for any assets and income you list - they usually require proof. If you're lucky, they'll just decline the loan if you can't provide proof that you have "foreign currency holdings" or an "offshore trust" to the value claimed. If you're less lucky, they'll regard it as attempted fraud.
