I will continue my research though, and might even take a leap of faith and still consider the rudder issue. If not connecting to the rudder but maybe using the rudder mount directly..
According to this article, the evolve puts out about 30# thrust, so presumably the rudder mount could handle this much as long as the prop stays relatively close to the mounts (less torque leverage against the mounts). http://www.louisianasportsman.com/details.php?id=2354
Keep in mind that if this damages your Oasis, it could be a costly non-warranty issue.
Here are a couple of other things to consider. The evolve has some important safety features:
-- If the pilot falls out of the boat, the motor shuts off, so the boat doesn't run away or cause lacerations.
-- if there is a capsize, the motor also shuts off.
-- the motor shuts down if it strikes an underwater object, restarts when clear.
However you rig a motor, I think these folks should have that kind of protection.
A rudder mount motor doesn't need a rudder as long as it's running, but definitely does need an attached rudder when not running (dead battery, malfunction, etc). The evolve rudder mount is adequate, but nowhere as good as the standard rudder for non-motorized running -- I would say that would be your minimum size requirement.
Putting the rudder mount motor in reverse tends to kick the motor out of the water. Locking your rudder down prevents that, but this is not a strong constraint. The evolve has a thrust limitation in reverse (I believe about 1/2 forward thrust)). You want to make sure any other rudder mount motor has a similar limiter.
These are some of the hurdles you need to consider if you still want to pursue a "custom" rudder mount. IMO, life would be much simpler if you could avoid it.
Here's another possibility. If your dealer had a "demonstrator" evolve and would be willing to loan it to you for these occasions, you can rig the system above the deck and avoid any holes in your boat (Velcro tape). Or even with holes, there are custom caps available to seal them with the system removed.