How to specify testnet via curl against json-rpc? The testnet=1 option can be (not) specified in the bitcoin.conf file (thus causing it to be on mainnet by default), and this can be over-ridden in bitcoin-cli with the -testnet command-line option.

But, when using the json-rpc interface via curl, I can't figure out how to specify I want testnet in this case. I can put the testnet port 18332 in the url, but I don't know how to use any other "command line" switches when using curl?

Can I do it with curl?
The port 18332 is the specification you speak of. 
When the testnet bitcoind is running on your system, port 18332 is open to accept rpc's. In case of mainnet, that port changes to 8332.

Every thing that you can do from the commandline is accessible via curl on that port.