Can Sandswept Tomb be used to discount Deflect as an additional cost if a Rampage is played on a unit there?

Yes, Sandswept Tomb's [A] discount can reduce the Deflect cost when playing Rampage — assuming the friendly unit you choose with Rampage is at Sandswept Tomb.

Here's how the cost stacking works (Rule 356):

1. Condition met — Rampage chooses a friendly unit. If that unit is at Sandswept Tomb, Rampage qualifies as "a spell that chooses one or more units here that are friendly to it" and gets the [A] discount.
2. Deflect is added first — Deflect is a mandatory additional cost (Rules 356.2.a.2, 809.1.d). When you target the enemy Deflect unit with Rampage, the Deflect cost is added to Rampage's total cost during step 3.2.
3. Sandswept Tomb applies after — Discounts are applied in step 3.4 (Rule 356.4). Per Rule 356.4.f, discounts can reduce additional costs, including to 0. So Sandswept Tomb's [A] off the total cost can offset part or all of the Deflect cost.

If the enemy unit has Deflect 1 ([A]), Sandswept Tomb would reduce that cost to 0. If it has Deflect 2 ([A][A]), Sandswept Tomb covers 1 of the 2 Power — you'd still owe the other [A].

Important caveat: Sandswept Tomb only cares about the friendly unit, not the enemy. If your friendly unit is elsewhere (not at Sandswept Tomb), none of this applies and you pay the full Deflect cost.