I was looking at the DVLA wording yesterday and although I couln't find anything about needing a vehicle identity check (VIC) after 'specifically' replacing the chassis (new for old), I think the words were 'If a vehicle has been substantially rebuilt', so I guess with the chassis being worth 5 out of the 8 points needed to keep the identity of the vehicle, then it could be interpreted as a 'substantial' replacement.
You'd be really unlucky to need a front half chassis as these things normally spew out enough oil up front to keep the forward rails in half decent shape.
I don't think the 'do one half now, one half later' thing would work, as the inspector won't inspect one half now then the other half later. He'll inspect the whole thing, whether you've done it over a period of time or not. Although, of course, the back of the chassis carries no ID.
There's also words in the DVLA stuff along the lines of 'Where there is evidence two vehicles have been welded together ....' which again could be difficult to dispute if a half-chassis has been used and there's a big seam down the middle of the chassis!