Ah gotcha.
I'd argue apple's ridiculously large foreign currency holdings are also a form of speculation (that at some point there will be another tax holiday like the one in 2004 that let corporations bring bring in foreign earnings at a 5% tax rate instead of a 35% tax rate). Until then they just sit. (And are not being used to pay dividends). So to me this seems more similar to overstock holding some of their non-USD income in the original currency they were paid in.
Regarding your concerns about how to carry bitcoins on a corporate balance sheet, a lot of bitcoin exchanges are legally recognized. "Legit" is more of a value judgement though, so I suppose one could argue none of them are legit. It'd be interesting to compare the volatility of the the bitcoin USD exchange rate to the variation in the price of commodities that are carried on corporate balance sheets. I agree with you it would be higher than the USD exchange rates for many or most government backed currencies. It is certainly lower than the volatility of some stocks, yet companies have no trouble carrying those on a balance sheet.
Implied questions get answered a lot less often that explicit questions. I honestly didn't realize you were curious about that, but now that I do:
I don't know what overstock is doing in this case specifically, but one option would be to use cryptographic techniques to split the private key into several "shards" which are held by different people. These can be constructed in such a way every piece is needed in order to use the private key, or that, for example any 5 of 7 pieces are needed to use the private key. This provides redundancy in the case of the loss of pieces of the key, while also not requiring complete trust in any one individual not to run off with the contents of the bitcoin wallet.