I am trying to work out what would be available to Imperial and Traitor forces late in the Heresy (approaching the Siege of Terra) which was not available earlier on.

So far I have come up with Power Armour up to Mark 7, and Vindicators. Terminator armour in the modern pattern is a possibility but not (to my knowledge) confirmed, as are assault cannons and multi-meltas.

Still not available are Razorbacks, Mark 8 armour, and a few Land Raider variants.

Can anyone think of anything else?

As an additional point, I think I read somewhere that the campaign badge for the Siege of Terra was the winged skull present on typical Mark 7 chest plates. Can anyone confirm a source for this?

Mk 7 armour (and possibly Mk 6) were not available until the latter stages of the Heresy. Mk 7 would have started becoming available just prior to the Siege of Terra.

The Vindicator started being used pretty early in the Heresy, but certainly was not available at the begginning.

Certainly Razorbacks are not available, and neither are any of the Land Raider variants (the earliest 'canon' introduction date is the Helios in M.38, although the Promethius may well have been developed much earlier - certainly none of the other variants were around before M.38).

Going by the artwork, some 'modern' Terminator suits had started being introduced, and although there is no other confirmation of this as far as I am aware, it certainly seems possible - witness the rapid development of power armour during this period and it seems likely new patterns of Terminator armour would have been developed, probably becoming available to the Loyalist Legions involved in the Siege of Terra.

Another development suggested by earlier background on Chaos Terminator weaponry is the Assault Cannon becoming available to Terminators (although it had long been available on Vehicles such as Dreadnoughts, Land Speeder Tornadoes and Baal Predators).

I haven't come across anything suggeesting that Multi Meltas were not available until later in the Heresy... have you got a source for this?

As for the Winged Skull on Mk 7, I can't recall a source for this. You may be confusing it with the studded pad on the Mk 6:
"Later the studded pad became associated with the Terran campaign and the final heroism of the Space Marines so that it became a traditional emblem of those days." (from the original Space Marine Armour background).

I also recall the winged skull being used as a campaign badge... is it in Codex:IG?

That might be it, Shroud.

I seem to recall a death guard toting a multi-melta and melta-ing a WE dreadnought in Collected Visions!