You can't just add them to the main Values list: they also need to be added to the VR sections (which are different parts of the form descriptor). You could - in theory - have a scheduled webhook that, running in the context of a dummy lead, calls the SFDC Tooling API endpoint to get the values, and writes the JSON response to a Marketo {{my.token}} using the Marketo REST API. (Or you could schedule the same thing with a 3rd-party scheduler, it's just handy to use the Smart Campaign scheduler.) Then output that {{my.token}} into a <script> block and merge the entries with the live form. A dev with experience in all these areas could definitely code it up, but would it be worth it... I dunno.
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