The Conduit API does indeed have a ping call. You can see it here:
http://www.multitech.net/developer/software/mtr-software/mtr-api-reference/command-table/
Based on the javascript used in the web interface, it looks like you need to pass it an ip field and an interface field. But that’s about all I know.
Otherwise, calling ping from Node.js may be as simple as using exec. Some great guidance is here:
https://stackoverflow.com/questions/4737130/how-to-ping-from-a-node-js-app
The payload is quite arbitrary. The BSD ping uses a 56 byte payload by default. The first 8 bytes is used for a timestamp so it can calculate round trip times. AFAIK, the rest is just garbage.