As a workaround, the value of a custom attribute can be XML.
So if a custom configuration profiles contains...

<key>allowCloudDocumentSync</key>
{{setting_allowCloudDocumentSync}}

Then the boolean preference "allowCloudDocumentSync" can be managed by setting the custom attribute "setting_allowCloudDocumentSync" to either "<true/> or "<false/>".

A custom attribute with data type "boolean" would be very helpful.
Currently, the only data type supported is "string", even if it can be marked as secret.

I would like to create a custom configuration profile that uses an attribute to populate a boolean field, like so:

<key>allowCloudDocumentSync</key>
<{{setting_allowCloudDocumentSync}}/>

In this case, the default value of the attribute would be "false", but I would be able to override that with "true" at the group or device level.

Currently, since the above is not valid Plist syntax SimpleMDM will reject the profile during upload or pasting into the console.