At BitMEX we do 100% offline multi-sig. Unfortunately, that also means any form of live proof-of-reserves is quite difficult to do, as the keys are offline.

Early in our history, we thought once-a-day withdrawal would be a problem customers would be quite vocal about. Instead, we found many of them appreciate what it means - that the keys aren't hooked up to a hackable webserver, but are actually protected. Accessing funds should be difficult and should require human interaction.