i've been thinking about this also.
first, with a rights offering, you have the existing shareholder base.
second, if the NWS senior prefs are wiped out and junior prefs are enticed to convert to common, you have a clean financing slate for new conventional senior pref that would be enticing to many institutional investors (pensions, insurance etc). but you are right, more is needed. but dont forget, if income rates go down, fnma can retain mucho dinero as long as dividends are foregone on all but any new senior prefs.
let me put it this way, mnuchin calls in wall st, and says to bankers he has about $1B in fees to dole out, my guess is that you will see the bushes being beaten (so to speak)
Buffett is on record about what he doesn't like - the non cumulative nature of preferreds, and the companies taking on too much risk in the past and focusing too much on quarterly earnings at the time he sold them. Berkshire certainly has the cash on hand to fire the "Elephant gun" and finance a very large part of the deal, the reinsurance market is well within their core circle of competence, it is a time when there are few undervalued investments around. This would certainly beat buying more of his least favorite industries in tech and airlines lol. The word lolapalooza effect comes to mind