We just got it working here! It might have been obvious but we are really new to all of this. So, turns out, mariaex already had support for Geometry. But to get it to work with Ecto, we needed to define a custom type.

defmodule MyApp.EctoPoint do
  @behaviour Ecto.Type
  def type, do: Mariaex.Geometry.Point

  # Casting from input into point struct
  def cast(value = %Mariaex.Geometry.Point{}), do: {:ok, value}
  def cast(_), do: :error

  # loading data from the database
  def load(data) do
    {:ok, data}
  end

  # dumping data to the database
  def dump(value = %Mariaex.Geometry.Point{}), do: {:ok, value}
  def dump(_), do: :error
end

Then, in your schema you can use that type. And in our case, we were always creating the point from user input that went into lat/long input fields, so in the changeset we cast those two to a proper Point.

defmodule MyApp.Restaurant do
  use Ecto.Schema

  import Ecto
  import Ecto.Changeset

  schema "pois" do
    # ... fields
    field(:lat, :float)
    field(:long, :float)
    field(:point, MyApp.EctoPoint) # <-- custom type here

    timestamps()
  end

  def changeset(%MyApp.Restaurant{} = restaurant, attrs \\ %{}) do
    poi
    |> cast(attrs, [:lat, :long])
    |> validate_required([:name])
    |> cast_geo()
  end

  def cast_geo(%Ecto.Changeset{changes: %{lat: lat, long: long}} = changeset) do
    point = %Mariaex.Geometry.Point{coordinates: {lat, long}}
    put_change(changeset, :geometry, point)
  end

  def cast_geo(changeset), do: changeset
end

Works like a charm! Only need to check what type to use in the migration.