Brightscript for brightsign and Roku diverged many years ago and components available in one may not be available in another. bright sign does not appear to have a 'roFilesystem' component.
I think 'Matchfiles' might be what you are looking for
ListDir(path As String) As Object
Returns an roList object containing the contents of the specified directory path. File names are converted to all lowercase.
MatchFiles(path As String, pattern_in As String) As Object
Takes a directory to look in (it can be as simple as "." or "/") and a pattern to be matched and then returns an roList containing the results. Each listed result contains only the part of the filename that is matched against the pattern, not the full path. The match is only applied in the specified directory; you will get no results if the pattern contains a directory separator. The pattern is a case insensitive wildcard expression. It may contain the following special characters:
? – Matches any single character.
* – Matches zero or more arbitrary characters.
[…] – Matches any single character specified within the brackets. The closing bracket is treated as a member of the character class if it immediately follows the opening bracket (i.e. "]" matches a single closed bracket). Within this class, "-" can be used to specify a range unless it is the first or last character (e.g. "[A-Cf-h"] is equivalent to "[ABCfgh]"). A character class may be negated by specifying "^" as the first character. To match a literal of this character, place it elsewhere in the class.
'roReadfile' may be useful
you could also try reading in the xml and see if you get an error
otherwise, ask in the brightsign community here