The ADL specifications imposes the following rule: "A type name is any identifier with an initial upper case letter, followed by any combination of letters, digits and underscores. A generic type name (including nested forms) additionally may include commas and angle brackets, but no spaces, and must be syntactically correct as per the UML. An attribute name is any identifier with an initial lower case letter, followed by any combination of letters, digits and underscores. Any convention that obeys this rule is allowed" (ADL 1.5 draft, page 26).
However, at the UML specifications I have only found the following style guidelines: "Capitalize the first letter of class names (if the character set supports uppercase)" and "Begin attribute and operation names with a lowercase letter". But I understand these as style recommendations and not as a mandatory specification since they are accompanied with others such as: "Center class name in boldface" and "Put the class name in italics if the class is abstract".
In any case, as we all know, object-oriented programming is not just UML. We can use other modeling tools or programming languages that do not impose the uppercase/lowercase rule. And moreover, at the AOM specifications I cannot find any reference about the fact that the rm_type_name String should begin with uppercase or the rm_attribute_name String with lowercase.
So, from a generic perspective of the dual modeling process, archetypes (or more specifically, ADL) should not impose rules in this aspect
Application of dual modeling and ADL syntax to other Reference Models other than openEHR