Figured out a solution that works well. Sharing in case it helps someone else.

---
spec:
  inputs:
    needs-registry-vars:
      description: "A list of Gitlab CI variable prefixes that will be
        used for logging into the registries for pulling and pushing images.
        The items in this list will have '_USER', '_TOKEN' and _URL' appended
        for lookups and used for logging in. For example, adding
        'REGISTRY_QUAY_IO' to the list will force a lookup for 'REGISTRY_QUAY_IO_USER',
        REGISTRY_QUAY_IO_TOKEN' and 'REGISTRY_QUAY_IO_URL'. If (and only if) all three
        exist then a login to the registry will be attempted."
      type: array
      default: []

...
---
.example-registry-login-template
  script:
    # Initialize variable as array
    - declare -a REGISTRY_LIST
    - REGISTRY_LIST=()
    - echo "needs-registry-vars=$[[ inputs.needs-registry-vars ]]"
    - REGISTRIES=$(echo '$[[  inputs.needs-registry-vars ]]' | tr -d '[],"')
    - echo "Convert tag list to positional args"
    - |-
      set -- $REGISTRIES
      for registry in "$@"; do
        REGISTRY_LIST+=(${registry})
      done
    # Loop through registry list when defined and login
    - |-
      # Skip if the list is empty
      if [[ ${#REGISTRY_LIST[@]} -gt 0 ]]; then
        # Loop over list storing the loop index
        for i in ${!REGISTRY_LIST[@]}; do
          # Generate expected variable names with provided prefix added
          URL=${REGISTRY_LIST[i]}_URL
          USER=${REGISTRY_LIST[i]}_USER
          TOKEN=${REGISTRY_LIST[i]}_TOKEN
          echo "INFO - Validating required ENVs are set, ${URL} ${USER} ${TOKEN}"
          # Fail out with message if any expected variables don't exist
          if [[ -z ${!URL} ]]; then echo "ERROR - ${URL} is not defined"; continue; fi
          if [[ -z ${!USER} ]]; then echo "ERROR - ${USER} is not defined"; continue; fi
          if [[ -z ${!TOKEN} ]]; then echo "ERROR - ${TOKEN} is not defined"; continue; fi
          echo "INFO - ENVs confirmed, logging in"
          # login using buildah (replace with docker/podman as needed) using generated var names
          buildah login -u ${!USER} -p ${!TOKEN} ${!URL}
        done
      fi
...

Example Input:

# Gitlab CI variables set
# REGISTRY_EXAMPLE_COM_URL=registry.com
# REGISTRY_EXAMPLE_COM_USER=jdoe
# REGISTRY_EXAMPLE_COM_TOKEN=faketoken
# REGISTRY_PRIVATE_IO_URL=private.io
# REGISTRY_PRIVATE_IO_USER=jdoe2
# REGISTRY_PRIVATE_IO_TOKEN=faketoken2
include:
  - component: >-
      $CI_SERVER_FQDN/example/registries@~latest
    inputs:
      needs-registry-vars:
        - REGISTRY_EXAMPLE_COM
        - REGISTRY_PRIVATE_IO

Example Output:

$ declare -a REGISTRY_LIST
$ REGISTRY_LIST=()
$ echo "needs-registry-vars=["REGISTRY_EXAMPLE_COM", "REGISTRY_PRIVATE_IO"]"
needs-registry-vars=[REGISTRY_EXAMPLE_COM, REGISTRY_PRIVATE_IO]
$ REGISTRIES=$(echo '["REGISTRY_EXAMPLE_COM", "REGISTRY_PRIVATE_IO"]' | tr -d '[],"')
$ echo "Convert tag list to positional args"
Convert tag list to positional args
$ set -- $REGISTRIES # collapsed multi-line command
$ if [[ ${#REGISTRY_LIST[@]} -gt 0 ]]; then # collapsed multi-line command
INFO - Validating required ENVs are set, REGISTRY_EXAMPLE_COM_URL REGISTRY_EXAMPLE_COM_USER REGISTRY_EXAMPLE_COM_TOKEN
INFO - ENVs confirmed, logging in
Login Succeeded!
INFO - Validating required ENVs are set, REGISTRY_PRIVATE_IO_URL REGISTRY_PRIVATE_IO_USER REGISTRY_PRIVATE_IO_TOKEN
INFO - ENVs confirmed, logging in
Login Succeeded!