If I play a weaponmaster unit as a reaction on an opponent's turn, can I equip an item?

Yes, you can equip an item.

According to RiftJudge FAQ #254 and Rule 821.1.b, Weaponmaster functions as a "When you play me" triggered ability. Because it is a triggered ability on play, it can be activated even when the unit is played during a reaction window (such as when playing a unit with Reaction on your opponent's turn).

The Sequence:

1. Play: You play your unit with Weaponmaster (e.g., using a [Reaction] to play it during your opponent's turn).
2. Trigger: The "When you play me" trigger for Weaponmaster is placed on the chain.
3. Resolution: When that trigger resolves, you may choose an equipment you control and pay its equip cost (reduced by [A]) to attach it to that unit, regardless of the usual timing for equipping.

Per Rule 821.1.c, this is an optional effect ("you may choose"). If you choose to do so, the attachment happens during the resolution of that triggered ability.