Guys, thanks for all of the replies. I tried to smooth out the top of the steerer, then used some steel wool to remove some of the rust. I got it past the top by hand but then it stopped again after an inch or so, most likely due to the remaining rust and the tight tolerances mentioned above.
So, I used a hammer to lightly bang down each side and it worked like a charm. Was it an elegant solution? Not at all. But it worked without breaking/bending the cap. I just hope I never need to get that thing off again!
p.s. I didn't see the lube recommendation until after I used the hammer.....next time.