Well... I'm not sure if US Airways "desperately needs RJs"... I think Air Wis desperately needs this deal in case they end up with no Major partners (remember, UAL, their only partner, put Air Wisconsin's services out to bid a few months ago... Also, remember AirWis operated AirTran JetConnect for about 18months as an attempt to diversify from UAL).
Also, I do not see this precluding a sale to Branson or anyone else. I would presume, since this is DIP financing, that it could be repaid at emergence from BK like "normal" DIP financing is...
In fact, this could be a good think for Branson, because he's only allowed a 49% stake in a US carrier (unless he has become a US citizen recently). As such, this would not preclude a Branson invenstment, and might actually help that cause by US Airways finding a partner for him (although I doubt that is what has happened... I think AirWis is desparate to ensure somebody will fly their aircraft...)
And yes... I am sure Johnnie O, who tried to do something similar by acquiring U stock through something called UFLY, won't be happy about it.