I understand perfectly what you mean.
I did something similar before but in my case it was really hard to accomplish because I needed some extract info from Hubspot without the API so it was hardcore game.
But a later update they fixed the only way to inject code in the previewer that I have found (in the labels of the fields), so for now I didn’t found any other way to accomplish it.
Anyway I will share with you some tracks about how I did it because maybe you find another way:
1- Get the info. Basic ajax to wherever you want, It was a hard step for me (because reasons that doesn’t apply to your case).
2- Insert the step1 in the custom module. Yep, I know that the preview is framed so you need something like that.
3- Create a function to re-build the field that you want with Select2 (which is the tech. that HS use to populate his selects).
4- Inject the code from the step 3, wrapped in the ready() function inside the label of any field of the Custom Module.
This last step is which doesn’t work anymore. So I don’t find the way to enhance the the previewer for now.
I hope you can!