I had a similar problem a few years ago, alloy frame and seat post. Ended up using the cutting with hacksaw method but had to do 4 cuts down the full length of the post then got an old chisel to carefully separate the frame and each section of post. Found that the one cut method and taking it out like a bushing didn’t work, nor did 2 cuts.
It took about 6hrs in total and was a bitch of a job, especially cutting evenly down the post and through it all – and making sure it was only post I was cutting. It won’t happen again. On the plus side the bike still gets ridden regularly so it was worth the effort.