You wrote "It's possible that either the seblod file transfer field or Joomla
itself prevents from uploading a zip file with some PHP code inside
(aside from the installation process of course) ?", I answered YES by pushing the link https://forum.joomla.org/viewtopic.php?t=965363
So you need a solution which is not using the Joomla Framework. You can certainly develop an upload SEBLOD field without using the upload control of Joomla or a add a custom code.