Not sure if this is the correct way, but I got it working w/ the following config.
...
"repositories": [
{
"type": "pear",
"url": "http://pear.php.net"
},
{
"type": "pear",
"url": "http://phpseclib.sourceforge.net",
"vendor-alias": "phpseclib"
}
],
"require-dev": {
"pear-pear/Mail": "*",
"phpseclib/Net_SSH2": "*"
}
...