I would like to practice against a particular opening - outside of bribing smart guinea pigs who know how to play chess, is there any way I can get opponents who always use a certain opening?
A great way of practicing your opening with a chess engine can be found on chess.com. You can create a free account, then go to the Learn Tab -> Book Openings -> Find the opening you want to practice -> Play Opening vs. Computer. You also have the chance to change the computer's difficulty level and what side you play.
You can set up any position and play it against computer at http://www.apronus.com/chess/wbeditor.php.
For example, you can play the Najdorf Sicilian as Black against computer.
You can adjust the strength of the computer by setting it time per move in milliseconds.
If you want human opponents to play specific openings, Chess.com theme tournaments are your best chances. Example of a themed tournament: "Sicilian-Dragon", where the initial position is setup in accordance with the opening. But these are not "live" chess but "online" chess where you'll be playing time controls like "1 move every 3 days" and such.
If you are OK to play against engines, then its really easy to setup the position (initial opening moves) in any GUI like Arena or SCID vs PC and then continue from there against the engine.
To add: There are some strong players on FICS who are willing to play any specific opening you want, you will have to befriend them and play against them. :-)