Playing DSi sav on Emulator?

Is it possible to use the Action Replay DSi to create a back up save of Pokemon Platinum, dump the back up on a pc and load it on an emulator like DeSmuMe or No$GBA?

Probably. You would have to convert the save file from the .DUC format to a .SAV format though. I would recommend using Shunyweb's online converter, but that is down I believe, and I don't know any other converter.

Well, I've tried Shunny's converter before, without any success, but if he's doing maintenance maybe it will work later.

Since Shuny's site is down, try using this one.
http://filetrip.net/f10337-Save-File-Type-Converter-1-8-Alpha.htmlBut, it can only convert some save types.