Ivory now supports custom instance emoji on  (Mac) and   (iOS/iPadOS)!

The update will roll out over the next few days, but you can grab it now here:
 https://apps.apple.com/us/app/ivory-for-mastodon-by-tapbots/id6444602274

Once updated, just tap the smiley face icon in the compose window to view and select a custom emoji from your server instance.