Verisimilitude is not limited to inflicting injuries and recovering from them. It includes a lot of things, many of which are not consciously considered in actual play, and (again) it is not strict simulation of the real world (in large part due to things like magic and superpowers) but rather using the real world as default and accommodating unreal things (like giant humanoid robots as viable war machines) as required.
Regarding the unreal, one must think of them as one would the creation of a tool or application. You start with the desired result. Then you work backwards to find the path that best accomplishes that result. This is the essence of competent design in general, especially so in gaming, and as such you should sear that into your memory and remember it always.
So, let's talk house rules and the unreal.
RIFTS has a lot of unreal elements. However, much of what is unreal has a basis in real world esoterica (and, frankly, is much improved when you modify it to better adhere to said sources- something Siembieda is loathe to do) and it is best that you become aware with this esoterica and how these real world unrealities are said to work before you start on the rule-crafting process. Ley-Lines, auspicious days and times, the use of mana to power magic, secret super-technologies, hidden worlds, false histories (in other words, what you usually see on History and H2 these days)- all good stuff to get familiar with when you're getting into messing with the unreal elements of the game.
While I covered playing various forms of unreal characters previously (and thus won't repeat myself here), I did not address becoming some form of unreal character. In practice, this usually means an otherwise ordinary character that is already in play changing into one that is not due to the integration of invasive cybertechnology or bionics such that it would trigger an Occupation change. However, this can also incorporate assumption of superhuman or supernatural power.
In this case, the rule should be something obvious: "The assumption of sufficient augmentative power--magic, technology, etc.--shall prompt the change of that character's Occupation to conform to their new state of being."