No. Seriously... I wouldn't. There's a lot of unsupported seat post extending out from a Brompton.

I'm not entirely sure why you want to lighten the Brompton that much. Even if you could switch out the seat post, you wouldn't save more than a pound: maybe two pounds at best. For a bike that weighs between 26 lb to 30 lb typically, the weight savings from swapping the seat post is negligible. Have you picked it up separately? I normally take out the seat post when I pack my Brompton for travel. Seriously... it's not that heavy.

By the time you get on the bike, load it up with whatever is in your C-bag (or whatever), the weight savings from the swapped seat post will be unnoticeable. For some reason, people seem obsessed with light weighting a bike. Unless you're racing, it's going to make barely any difference. To paraphrase Grant Petersen of Rivendell fame... most cyclists are better off losing some weight off the engine (you, me, whoever), not the bike. A couple of pounds lost for most people is more than tolerable and probably preferable. A couple of pounds off a bike means you might be compromising structural integrity.