AA, no doubt vested folks are going to exert to protect their interests as they see them. In this case, as normally is the case of liquidation of real property the order with which folks are made whole is:
1. tax man
2. first mortgage
3. everybody else (e.g. 2nd mortgage, HELOC, etc)
4. "owner"
as energy improvements are generally "home improvement" and home improvement normally gets financed as the level of #3 above, and by migrating that to #1 above, Fannie and Freddie felt leap-frogged.
The ultimate back-stop for Freddie and Fannie is the federal taxpayer, whereas the property tax (#1) is local.
Unfortunately, what is likely the case is that the homes and property that would take advantage of this opportunity are also the properties that would likely never be foreclosed ... so one would hope that Freddie and Fannie did actuarials or whatever they call it on risk associated with property undergoing energy efficiency upgrades versus property in general ... and chill out, indeed such improvements would likely make owners feel MORE vested.