Here’s this morning’s response from Readdle:
Thank you so much for your emails and provided information. Let me apologize for the misunderstanding.
I’ve just double-checked the information with our Quality Assurance team and would like to let you know the URL scheme, indeed, is available in Spark. However, the new line characters in the body text feature is not supported at the moment.
Please be assured that I’ve added your kind request to our Wish List for the developers to look through and keep in mind for further investigation of the matter. Once there is any update on the matter - I’ll be happy to let you know.
Also, if you have the Spark app set as your default mail client on your device, please consider using the mailto: link, as it will work the same way as the URL scheme.
For example, "mailto:one@test.email,two@test.email?subject=test&body=some%0Atext&cc=three@test.email", where %0A is the new line character.
Please don’t hesitate to contact me back if there is anything else I could do for you.
So, apparently they are recommending using mailto: instead of the Spark URL scheme.