Your most straightforward solution is to maintain your options in the form itself
<select id="structureAccueil"
class="form-control"
cam-variable-name="structure"
cam-variable-type="String">
<option value="001">DRH</option>
<option value="002">DKO</option>
<option value="003">DDD</option>
</select>
But I’m guessing you’re ultimately looking for something more dynamic. As you indicated, I think your options are pretty limited, given the nature of the start form. The only option I can think of is to load the data via a REST call back to the server, similar to what is described in this thread. Keeping that data as a resource in the deployment seems a little heavy-handed, but you could maintain your own API for something more lightweight.