Yes, this was added in Assure 9.2. For Web Service and External Service layouts, which includes the REST API layout, you can use the following syntax for your endpoint URL to reference a custom property file:
${@userprop_t('restapi.properties','base_rest_api_endpoint')}/api/v2/some_endpoint/
The expression is contained within ${ } and can be embedded within a text value.
For user ID and passwords, you can use the Edit Network Credentials option within a Layout to create expressions via the UI. The ${ } notation isn't needed here, and the same type of logic to reference property files via userprop_t can be used.