When setting an ADMX Action Policy Value, text boxes have a 1023 character limit. This limit doesn't apply when applying an ADMX through GPO/local policy.
Environment Manager Group Policy adheres to the Microsoft Max EditText Length for ADM(X) files.
However Group Policy does not appear to follow the 1023 character limit, therefore it would be possible to set the action using either Local Policy or Group Policy.
If this is not suitable, it may be possible to determine the registry value modified by the ADMX action and setting the registry value manually. The ADMX files used by the EM Console are typically stored in %SystemRoot%\PolicyDefinitions (e.g. c:\windows\PolicyDefinitions). These are XML files and can be viewed using a standard text editor. The ADMX file will list both the Policy Name and the Registry value that is modifed by the Policy.
This may not be a suitable solution for all Group Policies, such as when a Client Side Extension is required.