This discussion prompted me to call Fidelity to clarify exactly what a margin account means. They have been getting lots of calls on the subject. Like you, I have a taxable brokerage account registered as 'margin,' ostensibly to allow for the bridging and overnight settling of buys and sells of stock to and from my money market fund to the core/sweep fund, but the rep verified for me that I have not taken the extra step to allow for lending of my BRK shares. That is what I was glad to hear.

Also, piggbacking off of rationalwalk's post, the direct registry thingy is something Fidelity did do for some GameStop investors last year. My rep said someone will get back to me to explain how to do it in a more granular way, but admitted that it is a cumbersome process, with some pros and cons, and may not offer all that much more peace of mind for the safety considerations being discussed here, given the headaches. Next, I got to call Schwab, where my 401k resides, and clarify the same issues.