Is it possible to use a ProxyFish Proxy with Mirror Mode? I understand it won't allow me to enter it into the client prior to loading up, but is there a way around this?

Proxifier?

Not sure if this is outdated or not but this might be of help to you.

 

 

 

You can use a command line to run the official client with a proxy (with IP auth, I'm not sure how to use proxies with username and password auth with this method):

"C:\Program Files (x86)\Java\jre1.8.0_121\bin\java.exe" -DsocksProxyHost=111.xxx.xxx* -DsocksProxyPort=xxxx* -jar -Xmx384m -Xss2m -Dsun.java2d.noddraw=true -XX:CompileThreshold=1500 -Xincgc -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -Dcom.jagex.config=http://oldschool18.runescape.com/jav_config.ws C:\Users\David\jagexcache\jagexlauncher\bin\jagexappletviewer.jar oldschool

*: enter proxy details