Yeah, after removing the T-case myself and R&R the seal, I have an update....
The plastic nose plate's geometry at the end of the actuator assembly is the cause of the leak

; not to mention it's held to the nose by only one sheet metal threaded screw. Over time the plate warps, thereby increasing a radial portion of the O-ring's groove. Then the O-ring is allowed to move axially (bad), which allows the 90W oil to slip by.
Replacing the O-ring only delays the occurance til later.
The plastic plate is the problematic root and must be addressed.
I lathed and milled my own *metallic* nose plate, with some increased thickness here and there. I have pictures but this beginner IH8MUD account won't let me post them. I also added a spring that pushes against the T-case female pocket bore AND conversely against my metallic plate, to compress the plate against the actuator's underlying nose. I also used a Viton O-ring instead of the Toyota stock neoprene ring which takes a set.
Now it's a test of time.

I should know for sure in 30,000 - 50,000 miles

if the re-design is appropriate. And a reasonable bearing and seal shop should be able to supply a new seal, albeit it will only be 7mm tall, instead of the stock 8mm height. If you want pictures, PM me or reply.