Alloy post well corroded into the steel donor frame for the front half on my tandem project. It's been in the vice (twisting the post), gallons of WD40 and diesel, frame heated, whacked with a lump hammer, but it won't budge.

Any ideas (other than a 32mm drill bit)?

is it the seatpost you want, or you're going to weld up a tandem using an old frame ❓ 😯

hacksaw a slot out of the seat pin and then it will be able to be got out

I want to keep the frame, the post is scrap. The corroded bit is stuck more solid than the torsion capacity of the post. I gripped the seatpost in my vice and turned the frame, but the post just twists.

OK, You'll be needing the might of SHELDON for this one I think

you could try drilling two holes on the post, putting a large bolt/bar thru' and putting that in the vice – it'll stop the post twisting.
Otherwise – Sheldon Brown…

Straight to Sheldon's point XVI then!

Liquid nitrogen can give enough thermal shock and freeze (shrink) the seatpost enough to get it out, holding seat post in a vice. An old carbon dioxide fire extinguisher can do the similar thing.
Or Loctite freeze and release
http://loctitefreezeandrelease.com/.
Wurth Rost off
http://www.sjscycles.co.uk/product-Wurth-Wurth-Rost-Off-Ice-400ml–0893240-19954.htm.
Ambersil rust flash.
etc

I tried all the above [except nitrogen!] but the only thing that worked was taking it to a bike shop with a torch and they literaly melted the post out and reamed the seat tube. fried the paint in that area but it needed painting anyway

Ahhh, bu66er. Snapped the seatpost. Seatpost in vice, heated frame up, turned frame – seatpost bust!

Looks like it's time to drill it out. I measured the bit that remains in the frame. It's 210mm, bloody shortarse previous owner.