Is there any chance that the author of HeidiSQL would consider including providing an ERD on the roadmap?
I realise there was a post on this 4 years ago, but maybe there are others who would want this. (And just maybe, your openness to it might have increased?)
Also - I wonder if there might be some other project that could provide a kickstart on this? Specifically, the software: schemaspy.sourceforge.net is licensed under the GNU Lesser library license, making it available for you to make use of with little barrier if you choose.
I would really love to see this ability become available within your product!