I am really not an expert on this stuff, but I'm putting my BTC in my Electrum wallet for the next couple weeks. I choose Electrum specifically because it allows me to access the private keys directly (of course, there are plenty of others that do as well). I'd have the same concern as you - if you don't have the PKs, and you want to get control of your bit of the various hypothetical BTC forks, how do you do it?
The MPK seems like it should let you generate the PKs directly, but I don't know exactly and I find the Electrum approach just more foolproof.
Having the "master private key" or really the seed for the hierarchical determination is the same as having your private keys.
A master key allows you to derive an unlimited number of private keys. The method for turning the 24 word seed into private keys is known, implemented and published in the form of BIP 32.
More information can be found here: https://en.bitcoin.it/wiki/Deterministic_wallet
The electrum approach is more foolproof but negates all the benefits of using a HD wallet in the first place - let alone the secureity offered by not needing to load your master private key into a computer.
That's kind of where I was leaning with all this too, only I was looking at the client wallet as the option to obtain private keys.
Electrum does seem more feasible though, thanks to not requiring to download all the block-chain transactions.
Thanks for your response :)