Michael,

It would appear that the proxy code itself exposes the methods that will allow you to do what you want. 

Here are some screen snips of the code in action;

Here is the code I used. 

First we need to call our new routine from the existing proxy token validation sub.  Any other validation subs from other tokens exposed in our activity will also need to call this same routine .

/// <summary>
/// Validates the 'MyToken' property by adding validation errors and/or warnings to the specified IValidationErrorCollection.
/// </summary>
protected virtual void ValidateProperty_MyToken(IValidationErrorCollection collection)
{
    // TODO: Add code for validating the MyToken property here. For example:
    // 
    // Ensure that MyToken is not null.  If it isnt null then call the routine to validate any tokens exposed...
    if ((string.IsNullOrEmpty(this.MyToken)))
        collection.AddError("This property must be specified.");
    else
        this.ValidateToken(collection, MyTokenPropertyName, this.MyToken);
}

The new sub is called 'ValidateToken'.  In that new sub we are going to get a list of tokens from other activities to parse.  The existing proxy method 'RetrieveTokenDataFromContextActivity' seems to provide only the tokens from other activities that are 'above' our activity in the workflow.  We are going to parse those tokens to make sure any tokens referenced in our activity are available.  If the tokens referenced in our activity are not found then add a warning to the proxy 'ValidationErrorCollection'.

/// <param name="collection">The iValidationErrorCollection.</param>
/// <param name="tokenPropertyName">The name of the token to validate</param>
/// <param name="tokenValue">The value of the token to validate.</param>
private void ValidateToken(IValidationErrorCollection collection, string tokenPropertyName, string tokenValue)
{

    // Step through each token string in the token value...
    foreach (Match tokenName in Regex.Matches(tokenValue, @"%\((.*?)\)"))
    {
        string unDelimitedTokenName = tokenName.Groups(1).ToString;
        bool tokenFound = false;

        // Step through each token in the token data...
        foreach (Token token in this.GetTokenService.RetrieveTokenDataFromContextActivity(this).GetAllTokens)
        {
            // If it matches then set the flag and exit the loop...
            if (token.FullName == unDelimitedTokenName)
            {
                tokenFound = true;
                break;
            }
        }

        // If the token was not found then add a warning...
        if ((!tokenFound))
            collection.AddWarning(string.Format("Token '%({0})' cannot be resolved in '{1}'.", unDelimitedTokenName, tokenPropertyName));
    }
}

Finally, the 'OnActivityMoved and 'OnActivityRemoved' stubs that were added when you check the checkboxes in the proxy generator can be modified by calling the existing 'RefreshValidation' method on the exposed workflow object;

/// <summary>
/// Occurs when an activity has been moved within a parent activity or moved to a different parent activity but stays in the same workflow.
/// </summary>
/// <param name="workflow">The workflow within which the activity was moved.</param>
/// <param name="activity">The activity that was moved.</param>
/// <param name="previousParent">The parent activity of the activity that was moved, before it was moved.</param>
/// <param name="previousIndex">The index (within the parent activity) of the activity, before it was moved.</param>
    public virtual void OnActivityMoved(IWorkflow workflow, IActivityProxyEx activity, ICompositeActivityProxyEx previousParent, int previousIndex)
    {
        workflow.RefreshValidation();
    }

///<summary>
///Occurs when an activity has been removed from a workflow.
///</summary>
///<param name="workflow">The workflow from which the activity was removed.</param>
///<param name="activity">The activity that was removed.</param>
///<param name="previousParent">The parent activity of the activity that was removed, before it was removed.</param>
///<param name="previousIndex">The index (within the parent activity) of the activity, before it was removed.</param>
    public virtual void OnActivityRemoved(IWorkflow workflow, IActivityProxyEx activity, ICompositeActivityProxyEx previousParent, int previousIndex)
    {
        workflow.RefreshValidation();
    }