If heat is going to be applied the barrel needs to be hotter than the piston; there are a couple of different ways of achieving this, eg. heat the barrel taking care not to also heat the piston (eg damp rag on top of it) or cool the piston (got any dry ice?) . Or both
I think Steve's pointing out that with enough heat you could just melt the top off the piston anyway!
That idea with the plate would work pretty well - as long as you can get a seal between the plate and the barrel.
(Putting some oil/diesel above the piston and heating it can help the fluid to penetrate. Also just heating up good and hot then allowing to cool a few times can help as the heating/cooling can break down some of the things causing the 'stiction')
Use something really heavy to hit the wood with - it's a lot easier to control and to get a good solid 'thump' against it, which is often the trick with something seized.
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)