Hi Sergio,
got this working, too. Quote from the Release Notes:
The parameter for the Layout Template can now either contain an integer value (1..6) to select one of the predefined templates - OR - (new:) it can directly contain a path to a Layout Template file. The same placeholders work here as in the predefined Layout Template filenames, so you can e.g. use:
{gpxtrackmap}MyTrack.gpx,tpl=%TEMPLATEDIR%/MyLayoutTemplate.html{/gpxtrackmap}
(Note: you MUST use forward slashes ("/") as directory separator, backslashes ("\") will not work!)
cheers, Frank