I started my bitcoin business small, and if I had to put out 100 BTC to some agency that kept it (not just a deposit) I would not have probably started. On top of that, 100 BTC is not a huge barrier to a true scammer if he was going to start taking deposits for underpriced 7990's for example.
The biggest thing that separates the scammers from the real businesses is time and physical addresses.
Of course it is just a deposit. The money is on the line in the event of a scam, but is refunded whenever the business wants provided that no scam has occurred yet. This refund is announced, so that the fact that the business is no longer "bonded" becomes known to consumers.
Most of these businesses are probably investing substantially in bitcoin on the side anyways. It should not matter for them that much if a trusted third-party holds their bitcoin savings to ensure good behavior.
Yes, the key is "transparency" . Basically, there would be a list of "bonded/insured" bitcoin business's who have "deposited" this amount. Like you said this is just a "deposit" that can be returned upon the company folding and thus announced to the community at large they aren't a "bonded/insured" company. I agree I thinking most legitimate business would have no problem fronting some bitcoin.
The biggest issue will be "fraud complaints" but this will have to come in the form of "class action lawsuits" sort of thing where a group of people will have to make the case not a single individual.