I was thinking about it. A lot in fact, and did not find good solution. The problem is that libbe has no concept of "alternate root", and implementing it means more complex changes in libbe. Then from another hand, flipping bootloader from one to another is something not to be taken lightly, out of sync grub menu.lst, missing capability/findroot config etc is making such approach really fragile and error prone.
The purpose of this mechanism is really about case when some distro needs to buy more time to actually perform the switch while still keeping up with the updates coming in from gate. Adjusting installers, reviweing operational procedures, updating documentation/wikis/helpdesk, whichever other items people have in place, most probably instate the need for such mechanism and delay.
The similar situation was also in Solaris 11 when they did switch to grub2 - while there is some internal support for old grub, the actual switch was one time action. Scenarios like this are one of the "dark" side of BE's sharing [partial] configuration.
So eventually I just opted to have separate checks in bootadm and libbe. Especially considering the fact, after some time, the grub bits should be removed.