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).
I'm sure you are aware that if you declare it to the bank then they will probably share information with the IRS or your national tax office, its probably very hard to 'undeclare' something and I'm sure there will be relevant taxes to pay on large sums of capital.
Perhaps "foreign currency holdings" is the way to do it. And I would actually agree with undervaluing it since there is a lot of volatility.
And the taxes would only be due when the BTC is exchanged for fiat, assuming there is a profit. And I can certainly understand that those who value the anonymity of BTC would never want to reveal their balance to anyone. But I could imagine that for some people, the potential benefits of getting a good loan would outweigh the risk of eventually paying taxes on the BTC profit.