Can I use Rell's ability when I attack into an empty battlefield?

No, you cannot use Rell, Magnetic's ability when moving into an empty battlefield.

According to FAQ #8664 and FAQ #8689, moving a unit to an empty battlefield initiates a non-combat Showdown, not a Combat. Because there is no opposing unit to create a Combat, your unit does not gain the "Attacker" designation.

Rell, Magnetic's ability is a triggered ability that reads: "When I attack, you may play an Equipment..." Since this trigger specifically requires the unit to be designated as an attacker, and the "Attacker" designation is only granted during Combat (Rule 437 and Rule 440), the ability will not trigger when moving to an empty battlefield.

Summary of why this interaction fails:

1. No Combat: Moving to an empty battlefield creates a non-combat Showdown, not a Combat.
2. No Attacker Designation: Because there is no Combat, your unit does not receive the "Attacker" designation.
3. Trigger Condition Not Met: The ability "When I attack" requires the unit to be an attacker. As confirmed by FAQ #2505, this does not trigger when moving to an empty battlefield.

Note: I could not fully verify this ruling against the retrieved rules.