Not exactly what you are after, but you could add a "show route" button by just submitting a URL to Google with the adresses as parameters.
Here is a basic string which will return a map and directions from Google.
https://maps.google.com/maps?saddr=<<bo.addr1>>+to:<<bo.addr2>>+to:<<bo.addr3>>+to:<<bo.addr4>>
If the addresses are multiple fields, then Add a +. I.e. <<bo.addr1>>+<<bo.addr1city>>+<<bo.addr1state>>
Here is some cost with hard wired destinations, just as a sample.
<a target="_blank"href="https://maps.google.com/maps?saddr=Scituate+MA+to:Boston+MA+to:Newton+MA+to:Weymouth+MA"><img style="border:0px;" src="http://images.webestools.com/buttons.php?frm=1&btn_type=60&txt=Update%20Map" onmouseover="this.src='http://images.webestools.com/buttons.php?frm=2&btn_type=60&txt=Update%20Map'" onmouseout="this.src='http://images.webestools.com/buttons.php?frm=1&btn_type=60&txt=Update%20Map';" alt="Update%20Map" /></a>