Convert "Tag to Filename" command and filenames beginning with a period character

I'm using the "Convert tag to Filename" command with the following: $validate(%title%,_)

This works great, except occasionally it will generate filenames beginning with a period character which get ignored by both Foobar and Kodi as "hidden" system files.

If the first character of the filename is a period, is there a way to have it converted to an underscore?

there is not automatism as the dot is a legal character for filenames.
But you could enclose the whole expression for the converter into a $replace() function, e.g.

$replace(%artist%-%title%,.,_)

Is there a way I can replace the dot character only if it is in the first position?

try as format string:
$regexp('.hello',^\.,_)
this leads to _hello
So instead of the '.hello' insert your list of fields.

Please note that I edited this post and added a \in front of the dot