What is the latest information on adding repositories, I'd like to be able to use Finnish Land Survey maps, but I haven't yet figured out the magic incantation needed for this?

GoogleMaps seems to work with the following:

[
"GoogleMaps",
{
"prefix": "GoogleMaps",
"remote_url": "http://mt0.google.com/vt/x=%(x)d&y=%(y)d&z=%(zoom)d"
}
],