The answer is the same in all cases. You can't use a relative path. You'll need to concatenate the variable part of the path name in front of the relative part to use the FollowHyperlink Method.
PS, I don't use hyperlink data types for this. This particular datatype is Jet/ACE specific. I just use a string or memo if the paths can be long. I generally keep the relative path or just the file name and concatenate the full path at run time. The FollowHyperlink method doesn't require a hyperlink data type. It works with a text field.