Hi Brew
I'm sure you've already tried, but just in case: It's not just grayed out because you haven't yet added any recipients or the subject is blank?
I can confirm that's not the case. In "guided access" the send button is available whereas in "single app mode" the send button remains greyed out (even when entered a recipient address). I guess this is restricted on purpose by apple. Problem of guided access method is, that on rebooting the ipad after the battery got completely empty, the ipad isn't in guided access anymore and all apps are available. Which isn't good if you want to use it as a kiosk.
If not, maybe it's a limitation that can be turned on/off somewhere in the iOS settings?
I was looking for such a setting when creating/editing a profile in Apple configurator 2. There is actually a checkbox "use in Mail only" under Mail payloads. Even if this is unchecked, sending email from shuttersnitch in single app mode is not possible.
Kind regards
Egon